diff --git a/NAMESPACE b/NAMESPACE
index 236fbec4be..470d47faf5 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -178,6 +178,7 @@ importFrom(dplyr,full_join)
 importFrom(dplyr,group_by)
 importFrom(dplyr,group_by_at)
 importFrom(dplyr,if_else)
+importFrom(dplyr,lag)
 importFrom(dplyr,mutate)
 importFrom(dplyr,n)
 importFrom(dplyr,n_distinct)
diff --git a/NEWS.md b/NEWS.md
index bc182ddae6..cd1d9e2dac 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -12,6 +12,8 @@
 
 ## Updates of Existing Functions
 
+- `group_var` (optional) parameter is added to `derive_var_trtemfl()` to derive `TRTEMFL` for AE data if the data are collected as one episode of AE with multiple lines. (#2302)
+
 - Templates for ADPC, ADPPK and ADPP are updated to handle urine records. (#2392)
 
 - `create_single_dose_dataset()` has been updated to error if the `lookup_table` contains duplicates. (#2247)
diff --git a/R/admiral-package.R b/R/admiral-package.R
index 8e9292f149..b1ab40a87e 100644
--- a/R/admiral-package.R
+++ b/R/admiral-package.R
@@ -6,7 +6,7 @@
 #'             desc distinct ends_with everything filter first full_join
 #'             group_by group_by_at if_else mutate n n_distinct na_if pull
 #'             rename rename_with row_number select semi_join slice starts_with
-#'             summarise summarise_all tibble tribble ungroup union
+#'             summarise summarise_all tibble tribble ungroup union lag
 #' @importFrom hms as_hms
 #' @importFrom lifecycle deprecate_warn deprecate_stop deprecated
 #' @importFrom lubridate %--% as_datetime ceiling_date date days duration
diff --git a/R/derive_var_trtemfl.R b/R/derive_var_trtemfl.R
index 5c827f13df..c12435bf44 100644
--- a/R/derive_var_trtemfl.R
+++ b/R/derive_var_trtemfl.R
@@ -45,12 +45,12 @@
 #'
 #' @param initial_intensity Initial severity/intensity or toxicity
 #'
-#'   This derivation assumes AE data collection method as single record per AE
-#'   with "initial" and "most extreme" severity/intensity recorded separately.
+#'   `initial_intensity` is ignored when `group_var` is specified.
 #'
-#'   If the argument is specified, events which start before treatment start and
-#'   end after treatment start (or are ongoing) and worsened (i.e., the
-#'   intensity is greater than the initial intensity), are flagged.
+#'   If this argument is specified and `group_var` is `NULL`, events which start
+#'   before treatment start and end after treatment start (or are ongoing) and
+#'   worsened (i.e., the intensity is greater than the initial intensity), are
+#'   flagged.
 #'
 #'   The values of the specified variable must be comparable with the usual
 #'   comparison operators. I.e., if the intensity is greater than the initial
@@ -72,6 +72,22 @@
 #'   *Permitted Values:* A symbol referring to a variable of the input dataset
 #'   or `NULL`
 #'
+#' @param group_var Grouping variable
+#'
+#'  If the argument is specified, it assumes that AEs are recorded as one episode
+#'  of AE with multiple lines using a grouping variable.
+#'
+#'  Events starting during treatment or before treatment and worsening afterward
+#'  are flagged. Once an AE record in a group is flagged, all subsequent records
+#'  in the treatment window are flagged regardless of severity.
+#'
+#'  *Permitted Values:* A symbol referring to a variable of the input dataset
+#'   or `NULL`
+#'
+#' @param subject_keys Variables to uniquely identify a subject.
+#'
+#'   A list of symbols created using `exprs()` is expected. This argument is only
+#'   used when `group_var` is specified.
 #'
 #' @details For the derivation of the new variable the following cases are
 #'   considered in this order. The first case which applies, defines the value
@@ -90,10 +106,15 @@
 #'       if `start_date` is on or after `trt_start_date` and `start_date` is on
 #'       or before `trt_end_date` + `end_window` days, it is set to `"Y"`,
 #'  - *event started before treatment and (possibly) worsened on treatment*:
-#'    - if `initial_intensity` and `intensity` is specified: if
-#'    `initial_intensity < intensity` and `start_date` is before
-#'    `trt_start_date` and `end_date` is on or after `trt_start_date` or
-#'    `end_date` is `NA`, it is set to `"Y"`.
+#'    - if `initial_intensity`, `intensity` is specified and `group_var` is not specified:
+#'      if `initial_intensity < intensity` and `start_date` is before `trt_start_date`
+#'      and `end_date` is on or after `trt_start_date` or `end_date` is `NA`, it
+#'      is set to `"Y"`;
+#'    - if `group_var` is specified:
+#'      if previous `intensity` < `intensity` and `start_date` is after `trt_start_date`
+#'      and `end_date` is on or after `trt_start_date` or `end_date` is `NA`, it
+#'      is set to `"Y"`;
+#'
 #'  - Otherwise it is set to `NA_character_`.
 #'
 #' @return The input dataset with the variable specified by `new_var` added
@@ -109,7 +130,7 @@
 #' library(dplyr, warn.conflicts = FALSE)
 #' library(lubridate)
 #'
-#' adae <- expected <- tribble(
+#' adae <- tribble(
 #'   ~USUBJID, ~ASTDTM,            ~AENDTM,            ~AEITOXGR, ~AETOXGR,
 #'   # before treatment
 #'   "1",      "2021-12-13T20:15", "2021-12-15T12:45", "1",       "1",
@@ -153,6 +174,54 @@
 #'   initial_intensity = AEITOXGR,
 #'   intensity = AETOXGR
 #' ) %>% select(ASTDTM, AENDTM, AEITOXGR, AETOXGR, TRTEM2FL)
+#'
+#' adae2 <- tribble(
+#'   ~USUBJID, ~ASTDTM, ~AENDTM, ~AEITOXGR, ~AETOXGR, ~AEGRPID,
+#'   # before treatment
+#'   "1", "2021-12-13T20:15", "2021-12-15T12:45", "1", "1", "1",
+#'   "1", "2021-12-14T20:15", "2021-12-14T22:00", "1", "3", "1",
+#'   # starting before treatment and ending during treatment
+#'   "1", "2021-12-30T20:15", "2022-01-14T01:23", "3", "3", "2",
+#'   "1", "2022-01-05T20:00", "2022-06-01T11:00", "3", "1", "2",
+#'   "1", "2022-01-10T20:15", "2022-01-11T01:23", "3", "2", "2",
+#'   "1", "2022-01-13T20:15", "2022-03-01T01:23", "3", "1", "2",
+#'   # starting during treatment
+#'   "1", "2022-01-01T12:00", "2022-01-02T23:25", "4", "4", "3",
+#'
+#'   # after treatment
+#'   "1", "2022-05-10T11:00", "2022-05-10T13:05", "2", "2", "4",
+#'   "1", "2022-05-10T12:00", "2022-05-10T13:05", "2", "2", "4",
+#'   "1", "2022-05-11T11:00", "2022-05-11T13:05", "2", "2", "4",
+#'   # missing dates
+#'   "1", "", "", "3", "4", "5",
+#'   "1", "2021-12-30T09:00", "", "3", "4", "5",
+#'   "1", "2021-12-30T11:00", "", "3", "3", "5",
+#'   "1", "", "2022-01-04T09:00", "3", "4", "5",
+#'   "1", "", "2021-12-24T19:00", "3", "4", "5",
+#'   "1", "", "2022-06-04T09:00", "3", "4", "5",
+#'   # without treatment
+#'   "2", "", "2021-12-03T12:00", "1", "2", "1",
+#'   "2", "2021-12-01T12:00", "2021-12-03T12:00", "1", "2", "2",
+#'   "2", "2021-12-06T18:00", "", "1", "2", "3"
+#' ) %>%
+#'   mutate(
+#'     STUDYID = "ABC12345",
+#'     ASTDTM = ymd_hm(ASTDTM),
+#'     AENDTM = ymd_hm(AENDTM),
+#'     TRTSDTM = if_else(USUBJID == "1", ymd_hm("2022-01-01T01:01"), ymd_hms("")),
+#'     TRTEDTM = if_else(USUBJID == "1", ymd_hm("2022-04-30T23:59"), ymd_hms(""))
+#'   )
+
+#' # derive TRTEMFL taking treatment end and worsening into account within a grouping variable
+#' derive_var_trtemfl(
+#'   adae2,
+#'   new_var = TRTEMFL,
+#'   trt_end_date = TRTEDTM,
+#'   end_window = 10,
+#'   intensity = AETOXGR,
+#'   group_var = AEGRPID
+#' ) %>% select(ASTDTM, AENDTM, AEITOXGR, AETOXGR, AEGRPID, TRTEMFL)
+
 derive_var_trtemfl <- function(dataset,
                                new_var = TRTEMFL,
                                start_date = ASTDTM,
@@ -162,31 +231,58 @@ derive_var_trtemfl <- function(dataset,
                                end_window = NULL,
                                ignore_time_for_trt_end = TRUE,
                                initial_intensity = NULL,
-                               intensity = NULL) {
+                               intensity = NULL,
+                               group_var = NULL,
+                               subject_keys = get_admiral_option("subject_keys")) {
+  # Convert inputs to symbols
   new_var <- assert_symbol(enexpr(new_var))
   start_date <- assert_symbol(enexpr(start_date))
   end_date <- assert_symbol(enexpr(end_date))
   trt_start_date <- assert_symbol(enexpr(trt_start_date))
-  trt_end_date <- assert_symbol(enexpr(trt_end_date), optional = TRUE)
+  trt_end_date <-
+    assert_symbol(enexpr(trt_end_date), optional = TRUE)
   assert_integer_scalar(end_window, subset = "non-negative", optional = TRUE)
   assert_logical_scalar(ignore_time_for_trt_end)
-  initial_intensity <- assert_symbol(enexpr(initial_intensity), optional = TRUE)
+  initial_intensity <-
+    assert_symbol(enexpr(initial_intensity), optional = TRUE)
   intensity <- assert_symbol(enexpr(intensity), optional = TRUE)
-  if (is.null(initial_intensity) && !is.null(intensity)) {
-    cli_abort(c(
-      "{.arg intensity} argument was specified but not {.arg initial_intensity}",
-      "Either both or none of them must be specified."
-    ))
-  }
-  if (!is.null(initial_intensity) && is.null(intensity)) {
-    cli_abort(c(
-      "{.arg initial_intensity} argument was specified but not {.arg intensity}",
-      "Either both or none of them must be specified."
-    ))
+  group_var <- assert_symbol(enexpr(group_var), optional = TRUE)
+
+  # group_var is not specified
+  # Check if both initial_intensity and intensity are provided
+  if (is.null(group_var)) {
+    if (is.null(initial_intensity) && !is.null(intensity)) {
+      cli_abort(c(
+        "{.arg intensity} argument was specified but not {.arg initial_intensity}",
+        "Either both or none of them must be specified."
+      ))
+    }
+    if (!is.null(initial_intensity) && is.null(intensity)) {
+      cli_abort(c(
+        "{.arg initial_intensity} argument was specified but not {.arg intensity}",
+        "Either both or none of them must be specified."
+      ))
+    }
+    # group_var is specified
+  } else {
+    if (!is.null(initial_intensity)) {
+      cli_warn(c(
+        "{.arg initial_intensity} argument is ignored when {.arg group_var} is specified",
+        "Please only specify one of them."
+      ))
+    }
+    if (is.null(subject_keys)) {
+      cli_abort(c(
+        "{.arg group_var} argument was specified but not {.arg subject_keys}",
+        "{.arg subject_keys} argument must be provided when {.arg group_var} is specified."
+      ))
+    }
+    assert_vars(subject_keys)
   }
-  assert_data_frame(
-    dataset,
-    required_vars = expr_c(
+
+  # Assert required variables
+  required_vars <-
+    expr_c(
       start_date,
       end_date,
       trt_start_date,
@@ -194,7 +290,12 @@ derive_var_trtemfl <- function(dataset,
       initial_intensity,
       intensity
     )
-  )
+  if (!is.null(group_var)) {
+    required_vars <- c(required_vars, group_var)
+  }
+  assert_data_frame(dataset, required_vars = required_vars)
+
+  # Assert date variables
   assert_date_var(dataset, var = !!start_date)
   assert_date_var(dataset, var = !!end_date)
   assert_date_var(dataset, var = !!trt_start_date)
@@ -202,6 +303,7 @@ derive_var_trtemfl <- function(dataset,
     assert_date_var(dataset, var = !!trt_end_date)
   }
 
+  # end window condition
   if (is.null(end_window)) {
     end_cond <- expr(TRUE)
   } else {
@@ -212,27 +314,89 @@ derive_var_trtemfl <- function(dataset,
       ))
     }
     if (ignore_time_for_trt_end) {
-      end_cond <- expr(is.na(!!trt_end_date) |
-        date(!!start_date) <= date(!!trt_end_date) + days(end_window))
+      end_cond <- expr(
+        (is.na(!!trt_end_date) |
+          date(!!start_date) <= date(!!trt_end_date) + days(end_window))
+      )
     } else {
-      end_cond <- expr(is.na(!!trt_end_date) | !!start_date <= !!trt_end_date + days(end_window))
+      end_cond <-
+        expr(
+          (is.na(!!trt_end_date) |
+            !!start_date <= !!trt_end_date + days(end_window))
+        )
     }
   }
 
+
+  # new_ae_cond: Y - new AE, N - AE exists before trt_start_date
+  new_ae_cond <- get_new_tmp_var(dataset)
+  if (is.null(group_var)) {
+    dataset <- dataset %>%
+      mutate(
+        !!new_ae_cond := if_else(!!start_date >= !!trt_start_date, "Y", "N")
+      )
+  } else {
+    dataset <- dataset %>%
+      derive_vars_merged(
+        dataset_add = dataset,
+        by_vars = expr_c(subject_keys, group_var),
+        order = exprs(!!start_date),
+        new_vars = exprs(!!new_ae_cond := "N"),
+        filter_add = !!start_date < !!trt_start_date,
+        mode = "last"
+      ) %>%
+      mutate(
+        !!new_ae_cond := if_else(is.na(!!new_ae_cond), "Y", "N")
+      )
+  }
+
   if (is.null(intensity)) {
     worsening_cond <- expr(FALSE)
   } else {
-    worsening_cond <-
-      expr(!!start_date < !!trt_start_date &
-        (!!initial_intensity < !!intensity | is.na(!!initial_intensity) | is.na(!!intensity)))
+    if (is.null(group_var)) {
+      worsening_cond <-
+        expr(
+          !!start_date < !!trt_start_date &
+            (!!initial_intensity < !!intensity | is.na(!!initial_intensity) |
+              is.na(!!intensity))
+        )
+    } else {
+      prev_intensity <- get_new_tmp_var(dataset)
+      worsen_date <- get_new_tmp_var(dataset)
+
+      dataset <- dataset %>%
+        arrange(USUBJID, !!group_var, !!start_date) %>%
+        group_by(USUBJID, !!group_var) %>%
+        mutate(
+          !!prev_intensity := lag(!!intensity),
+          !!worsen_date :=
+            case_when(
+              !is.na(!!start_date) & !is.na(!!trt_start_date) & !is.na(!!prev_intensity) &
+                !!start_date >= !!trt_start_date &
+                (!!intensity > !!prev_intensity) ~ !!start_date,
+              TRUE ~ NA
+            )
+        ) %>%
+        fill(!!worsen_date, .direction = "down") %>%
+        ungroup()
+
+      worsening_cond <- expr(!is.na(!!worsen_date))
+    }
   }
 
-  dataset %>%
-    mutate(!!new_var := case_when(
-      is.na(!!trt_start_date) ~ NA_character_,
-      !!end_date < !!trt_start_date ~ NA_character_,
-      is.na(!!start_date) ~ "Y",
-      !!start_date >= !!trt_start_date & !!end_cond ~ "Y",
-      !!worsening_cond ~ "Y"
-    ))
+
+  # Derive TRTEMFL based on conditions
+
+  dataset <- dataset %>%
+    mutate(
+      !!new_var := case_when(
+        is.na(!!trt_start_date) ~ NA_character_,
+        !!end_date < !!trt_start_date ~ NA_character_,
+        is.na(!!start_date) ~ "Y",
+        !!new_ae_cond == "Y" & !!end_cond ~ "Y", # new AE
+        !!worsening_cond ~ "Y" # worsened AE
+      )
+    ) %>%
+    # Remove temporary variable
+    remove_tmp_vars()
 }
diff --git a/R/globals.R b/R/globals.R
index b498e406ab..f37003b299 100644
--- a/R/globals.R
+++ b/R/globals.R
@@ -129,5 +129,9 @@ globalVariables(c(
   "atoxgr_criteria_ctcv4",
   "DTYPE",
   "where", # this entry should be moved to @importFrom tidyselect once we use tidyselect 1.2.0
-  "tmp_col_type"
+  "tmp_col_type",
+  "num_records",
+  "worsen_date",
+  "prev_intensity",
+  "srfl"
 ))
diff --git a/man/derive_var_trtemfl.Rd b/man/derive_var_trtemfl.Rd
index aa8ae90b46..319a6006a4 100644
--- a/man/derive_var_trtemfl.Rd
+++ b/man/derive_var_trtemfl.Rd
@@ -14,7 +14,9 @@ derive_var_trtemfl(
   end_window = NULL,
   ignore_time_for_trt_end = TRUE,
   initial_intensity = NULL,
-  intensity = NULL
+  intensity = NULL,
+  group_var = NULL,
+  subject_keys = get_admiral_option("subject_keys")
 )
 }
 \arguments{
@@ -57,12 +59,12 @@ the event occurred more than \code{end_window} days after end of treatment.
 
 \item{initial_intensity}{Initial severity/intensity or toxicity
 
-This derivation assumes AE data collection method as single record per AE
-with "initial" and "most extreme" severity/intensity recorded separately.
+\code{initial_intensity} is ignored when \code{group_var} is specified.
 
-If the argument is specified, events which start before treatment start and
-end after treatment start (or are ongoing) and worsened (i.e., the
-intensity is greater than the initial intensity), are flagged.
+If this argument is specified and \code{group_var} is \code{NULL}, events which start
+before treatment start and end after treatment start (or are ongoing) and
+worsened (i.e., the intensity is greater than the initial intensity), are
+flagged.
 
 The values of the specified variable must be comparable with the usual
 comparison operators. I.e., if the intensity is greater than the initial
@@ -83,6 +85,23 @@ intensity \code{initial_intensity < intensity} must evaluate to \code{TRUE}.
 
 \emph{Permitted Values:} A symbol referring to a variable of the input dataset
 or \code{NULL}}
+
+\item{group_var}{Grouping variable
+
+If the argument is specified, it assumes that AEs are recorded as one episode
+of AE with multiple lines using a grouping variable.
+
+Events starting during treatment or before treatment and worsening afterward
+are flagged. Once an AE record in a group is flagged, all subsequent records
+in the treatment window are flagged regardless of severity.
+
+\emph{Permitted Values:} A symbol referring to a variable of the input dataset
+or \code{NULL}}
+
+\item{subject_keys}{Variables to uniquely identify a subject.
+
+A list of symbols created using \code{exprs()} is expected. This argument is only
+used when \code{group_var} is specified.}
 }
 \value{
 The input dataset with the variable specified by \code{new_var} added
@@ -111,10 +130,14 @@ or before \code{trt_end_date} + \code{end_window} days, it is set to \code{"Y"},
 }
 \item \emph{event started before treatment and (possibly) worsened on treatment}:
 \itemize{
-\item if \code{initial_intensity} and \code{intensity} is specified: if
-\code{initial_intensity < intensity} and \code{start_date} is before
-\code{trt_start_date} and \code{end_date} is on or after \code{trt_start_date} or
-\code{end_date} is \code{NA}, it is set to \code{"Y"}.
+\item if \code{initial_intensity}, \code{intensity} is specified and \code{group_var} is not specified:
+if \code{initial_intensity < intensity} and \code{start_date} is before \code{trt_start_date}
+and \code{end_date} is on or after \code{trt_start_date} or \code{end_date} is \code{NA}, it
+is set to \code{"Y"};
+\item if \code{group_var} is specified:
+if previous \code{intensity} < \code{intensity} and \code{start_date} is after \code{trt_start_date}
+and \code{end_date} is on or after \code{trt_start_date} or \code{end_date} is \code{NA}, it
+is set to \code{"Y"};
 }
 \item Otherwise it is set to \code{NA_character_}.
 }
@@ -125,7 +148,7 @@ library(tibble)
 library(dplyr, warn.conflicts = FALSE)
 library(lubridate)
 
-adae <- expected <- tribble(
+adae <- tribble(
   ~USUBJID, ~ASTDTM,            ~AENDTM,            ~AEITOXGR, ~AETOXGR,
   # before treatment
   "1",      "2021-12-13T20:15", "2021-12-15T12:45", "1",       "1",
@@ -169,6 +192,52 @@ derive_var_trtemfl(
   initial_intensity = AEITOXGR,
   intensity = AETOXGR
 ) \%>\% select(ASTDTM, AENDTM, AEITOXGR, AETOXGR, TRTEM2FL)
+
+adae2 <- tribble(
+  ~USUBJID, ~ASTDTM, ~AENDTM, ~AEITOXGR, ~AETOXGR, ~AEGRPID,
+  # before treatment
+  "1", "2021-12-13T20:15", "2021-12-15T12:45", "1", "1", "1",
+  "1", "2021-12-14T20:15", "2021-12-14T22:00", "1", "3", "1",
+  # starting before treatment and ending during treatment
+  "1", "2021-12-30T20:15", "2022-01-14T01:23", "3", "3", "2",
+  "1", "2022-01-05T20:00", "2022-06-01T11:00", "3", "1", "2",
+  "1", "2022-01-10T20:15", "2022-01-11T01:23", "3", "2", "2",
+  "1", "2022-01-13T20:15", "2022-03-01T01:23", "3", "1", "2",
+  # starting during treatment
+  "1", "2022-01-01T12:00", "2022-01-02T23:25", "4", "4", "3",
+
+  # after treatment
+  "1", "2022-05-10T11:00", "2022-05-10T13:05", "2", "2", "4",
+  "1", "2022-05-10T12:00", "2022-05-10T13:05", "2", "2", "4",
+  "1", "2022-05-11T11:00", "2022-05-11T13:05", "2", "2", "4",
+  # missing dates
+  "1", "", "", "3", "4", "5",
+  "1", "2021-12-30T09:00", "", "3", "4", "5",
+  "1", "2021-12-30T11:00", "", "3", "3", "5",
+  "1", "", "2022-01-04T09:00", "3", "4", "5",
+  "1", "", "2021-12-24T19:00", "3", "4", "5",
+  "1", "", "2022-06-04T09:00", "3", "4", "5",
+  # without treatment
+  "2", "", "2021-12-03T12:00", "1", "2", "1",
+  "2", "2021-12-01T12:00", "2021-12-03T12:00", "1", "2", "2",
+  "2", "2021-12-06T18:00", "", "1", "2", "3"
+) \%>\%
+  mutate(
+    STUDYID = "ABC12345",
+    ASTDTM = ymd_hm(ASTDTM),
+    AENDTM = ymd_hm(AENDTM),
+    TRTSDTM = if_else(USUBJID == "1", ymd_hm("2022-01-01T01:01"), ymd_hms("")),
+    TRTEDTM = if_else(USUBJID == "1", ymd_hm("2022-04-30T23:59"), ymd_hms(""))
+  )
+# derive TRTEMFL taking treatment end and worsening into account within a grouping variable
+derive_var_trtemfl(
+  adae2,
+  new_var = TRTEMFL,
+  trt_end_date = TRTEDTM,
+  end_window = 10,
+  intensity = AETOXGR,
+  group_var = AEGRPID
+) \%>\% select(ASTDTM, AENDTM, AEITOXGR, AETOXGR, AEGRPID, TRTEMFL)
 }
 \seealso{
 OCCDS Functions: 
diff --git a/tests/testthat/_snaps/derive_var_trtemfl.md b/tests/testthat/_snaps/derive_var_trtemfl.md
index fe6149e62f..2c3c488096 100644
--- a/tests/testthat/_snaps/derive_var_trtemfl.md
+++ b/tests/testthat/_snaps/derive_var_trtemfl.md
@@ -1,4 +1,4 @@
-# derive_var_trtemfl Test 4: error if `end_window` without `trt_end_date`
+# derive_var_trtemfl Test 6: error if `end_window` without `trt_end_date`
 
     Code
       derive_var_trtemfl(adae, end_window = 10)
@@ -7,7 +7,7 @@
       ! `end_window` argument was specified but not `trt_end_date`
       Either both or none of them must be specified.
 
-# derive_var_trtemfl Test 5: error if `initial_intensity` without `intensity`
+# derive_var_trtemfl Test 7: error if `initial_intensity` without `intensity`
 
     Code
       derive_var_trtemfl(adae, initial_intensity = AEITOXGR)
@@ -16,7 +16,7 @@
       ! `initial_intensity` argument was specified but not `intensity`
       Either both or none of them must be specified.
 
-# derive_var_trtemfl Test 6: error if `intensity` without `initial_intensity`
+# derive_var_trtemfl Test 8: error if `intensity` without `initial_intensity`
 
     Code
       derive_var_trtemfl(adae, intensity = AETOXGR)
@@ -25,3 +25,12 @@
       ! `intensity` argument was specified but not `initial_intensity`
       Either both or none of them must be specified.
 
+# derive_var_trtemfl Test 9: error if `intensity` without `initial_intensity`
+
+    Code
+      derive_var_trtemfl(adae2, intensity = AETOXGR)
+    Condition
+      Error in `derive_var_trtemfl()`:
+      ! `intensity` argument was specified but not `initial_intensity`
+      Either both or none of them must be specified.
+
diff --git a/tests/testthat/test-derive_var_trtemfl.R b/tests/testthat/test-derive_var_trtemfl.R
index b75b9950cb..c110da5730 100644
--- a/tests/testthat/test-derive_var_trtemfl.R
+++ b/tests/testthat/test-derive_var_trtemfl.R
@@ -34,6 +34,45 @@ expected <- tibble::tribble(
 
 adae <- select(expected, -starts_with("TRTEM"))
 
+expected2 <- tribble(
+  ~USUBJID, ~ASTDTM, ~AENDTM, ~AEITOXGR, ~AETOXGR, ~AEGRPID, ~TRTEMFL, ~TRTEM2FL,
+  # before treatment
+  "1", "2021-12-13T20:15", "2021-12-15T12:45", "1", "1", "1", NA, NA,
+  "1", "2021-12-14T20:15", "2021-12-14T22:00", "1", "3", "1", NA, NA,
+  # starting before treatment and ending during treatment
+  "1", "2021-12-30T20:15", "2022-01-14T01:23", "3", "3", "2", NA, NA,
+  "1", "2022-01-05T20:00", "2022-06-01T11:00", "3", "1", "2", NA, NA,
+  "1", "2022-01-10T20:15", "2022-01-11T01:23", "3", "2", "2", "Y", "Y",
+  "1", "2022-01-13T20:15", "2022-03-01T01:23", "3", "1", "2", "Y", "Y",
+  # starting during treatment
+  "1", "2022-01-01T12:00", "2022-01-02T23:25", "4", "4", "3", "Y", "Y",
+
+  # after treatment
+  "1", "2022-05-10T11:00", "2022-05-10T13:05", "2", "2", "4", "Y", "Y",
+  "1", "2022-05-10T12:00", "2022-05-10T13:05", "2", "2", "4", "Y", "Y",
+  "1", "2022-05-11T11:00", "2022-05-11T13:05", "2", "2", "4", NA, NA,
+  # missing dates
+  "1", "", "", "3", "4", "5", "Y", "Y",
+  "1", "2021-12-30T09:00", "", "3", "4", "5", NA, NA,
+  "1", "2021-12-30T11:00", "", "3", "3", "5", NA, NA,
+  "1", "", "2022-01-04T09:00", "3", "4", "5", "Y", "Y",
+  "1", "", "2021-12-24T19:00", "3", "4", "5", NA, NA,
+  "1", "", "2022-06-04T09:00", "3", "4", "5", "Y", "Y",
+  # without treatment
+  "2", "", "2021-12-03T12:00", "1", "2", "1", NA, NA,
+  "2", "2021-12-01T12:00", "2021-12-03T12:00", "1", "2", "2", NA, NA,
+  "2", "2021-12-06T18:00", "", "1", "2", "3", NA, NA
+) %>%
+  mutate(
+    STUDYID = "ABC12345",
+    ASTDTM = lubridate::ymd_hm(ASTDTM),
+    AENDTM = lubridate::ymd_hm(AENDTM),
+    TRTSDTM = if_else(USUBJID != "2", lubridate::ymd_hm("2022-01-01T01:01"), ymd_hms("")),
+    TRTEDTM = if_else(USUBJID != "2", lubridate::ymd_hm("2022-04-30T23:59"), ymd_hms(""))
+  )
+
+adae2 <- select(expected2, -starts_with("TRTEM"))
+
 ## Test 1: end_window and worsening parameters not specfied ----
 test_that("derive_var_trtemfl Test 1: end_window and worsening parameters not specfied", {
   expect_dfs_equal(
@@ -59,8 +98,25 @@ test_that("derive_var_trtemfl Test 2: with end_window and worsening", {
   )
 })
 
-## Test 3: considering trt end time ----
-test_that("derive_var_trtemfl Test 3: considering trt end time", {
+## Test 3: with end_window and worsening within grouping variable----
+test_that("derive_var_trtemfl Test 3: with end_window and worsening within grouping variable", {
+  expect_dfs_equal(
+    base = select(expected2, -TRTEM2FL),
+    comp = derive_var_trtemfl(
+      adae2,
+      new_var = TRTEMFL,
+      trt_end_date = TRTEDTM,
+      end_window = 10,
+      intensity = AETOXGR,
+      group_var = AEGRPID
+    ),
+    keys = c("USUBJID", "ASTDTM", "AENDTM")
+  )
+})
+
+
+## Test 4: considering trt end time without grouping variable----
+test_that("derive_var_trtemfl Test 4: considering trt end time", {
   expect_dfs_equal(
     base = select(expected, -TRTEMFL, -TRTEM2FL),
     comp = derive_var_trtemfl(
@@ -76,8 +132,25 @@ test_that("derive_var_trtemfl Test 3: considering trt end time", {
   )
 })
 
-## Test 4: error if `end_window` without `trt_end_date` ----
-test_that("derive_var_trtemfl Test 4: error if `end_window` without `trt_end_date`", {
+## Test 5: considering trt end time with grouping variable----
+test_that("derive_var_trtemfl Test 5: considering trt end time", {
+  expect_dfs_equal(
+    base = select(expected2, -TRTEMFL),
+    comp = derive_var_trtemfl(
+      adae2,
+      new_var = TRTEM2FL,
+      trt_end_date = TRTEDTM,
+      end_window = 10,
+      ignore_time_for_trt_end = FALSE,
+      intensity = AETOXGR,
+      group_var = AEGRPID
+    ),
+    keys = c("USUBJID", "ASTDTM", "AENDTM")
+  )
+})
+
+## Test 6: error if `end_window` without `trt_end_date` ----
+test_that("derive_var_trtemfl Test 6: error if `end_window` without `trt_end_date`", {
   expect_snapshot(
     derive_var_trtemfl(
       adae,
@@ -87,8 +160,8 @@ test_that("derive_var_trtemfl Test 4: error if `end_window` without `trt_end_dat
   )
 })
 
-## Test 5: error if `initial_intensity` without `intensity` ----
-test_that("derive_var_trtemfl Test 5: error if `initial_intensity` without `intensity`", {
+## Test 7: error if `initial_intensity` without `intensity` ----
+test_that("derive_var_trtemfl Test 7: error if `initial_intensity` without `intensity`", {
   expect_snapshot(
     derive_var_trtemfl(
       adae,
@@ -98,8 +171,8 @@ test_that("derive_var_trtemfl Test 5: error if `initial_intensity` without `inte
   )
 })
 
-## Test 6: error if `intensity` without `initial_intensity` ----
-test_that("derive_var_trtemfl Test 6: error if `intensity` without `initial_intensity`", {
+## Test 8: error if `intensity` without `initial_intensity` ----
+test_that("derive_var_trtemfl Test 8: error if `intensity` without `initial_intensity`", {
   expect_snapshot(
     derive_var_trtemfl(
       adae,
@@ -108,3 +181,26 @@ test_that("derive_var_trtemfl Test 6: error if `intensity` without `initial_inte
     error = TRUE
   )
 })
+
+## Test 9: error if `intensity` without `initial_intensity` ----
+test_that("derive_var_trtemfl Test 9: error if `intensity` without `initial_intensity`", {
+  expect_snapshot(
+    derive_var_trtemfl(
+      adae2,
+      intensity = AETOXGR
+    ),
+    error = TRUE
+  )
+})
+
+## Test 10: warning if both `initial_intensity` and `group_var` are specified ----
+test_that("derive_var_trtemfl Test 9: error if `intensity` without `initial_intensity`", {
+  expect_warning(
+    derive_var_trtemfl(
+      adae2,
+      initial_intensity = AETOXGR,
+      group_var = AEGRPID
+    ),
+    "`initial_intensity` argument is ignored when `group_var` is specified"
+  )
+})