diff --git a/lib/deprecatedlib.php b/lib/deprecatedlib.php
index 5116da890b5eb..7751186ad3d4f 100644
--- a/lib/deprecatedlib.php
+++ b/lib/deprecatedlib.php
@@ -4100,3 +4100,29 @@ function enrol_cohort_search_cohorts(course_enrolment_manager $manager, $offset
     }
     return array('more' => !(bool)$limit, 'offset' => $offset, 'cohorts' => $cohorts);
 }
+
+/**
+ * Is $USER one of the supplied users?
+ *
+ * $user2 will be null if viewing a user's recent conversations
+ *
+ * @deprecated since Moodle 2.9 MDL-49371 - please do not use this function any more.
+ * @todo MDL-49290 This will be deleted in Moodle 3.1.
+ * @param stdClass the first user
+ * @param stdClass the second user or null
+ * @return bool True if the current user is one of either $user1 or $user2
+ */
+function message_current_user_is_involved($user1, $user2) {
+    global $USER;
+
+    debugging('message_current_user_is_involved() is deprecated, please do not use this function.', DEBUG_DEVELOPER);
+
+    if (empty($user1->id) || (!empty($user2) && empty($user2->id))) {
+        throw new coding_exception('Invalid user object detected. Missing id.');
+    }
+
+    if ($user1->id != $USER->id && (empty($user2) || $user2->id != $USER->id)) {
+        return false;
+    }
+    return true;
+}
diff --git a/message/lib.php b/message/lib.php
index 81d6609de5716..92e06dd463c15 100644
--- a/message/lib.php
+++ b/message/lib.php
@@ -2617,28 +2617,6 @@ function message_page_type_list($pagetype, $parentcontext, $currentcontext) {
     return array('messages-*'=>get_string('page-message-x', 'message'));
 }
 
-/**
- * Is $USER one of the supplied users?
- *
- * $user2 will be null if viewing a user's recent conversations
- *
- * @param stdClass the first user
- * @param stdClass the second user or null
- * @return bool True if the current user is one of either $user1 or $user2
- */
-function message_current_user_is_involved($user1, $user2) {
-    global $USER;
-
-    if (empty($user1->id) || (!empty($user2) && empty($user2->id))) {
-        throw new coding_exception('Invalid user object detected. Missing id.');
-    }
-
-    if ($user1->id != $USER->id && (empty($user2) || $user2->id != $USER->id)) {
-        return false;
-    }
-    return true;
-}
-
 /**
  * Get messages sent or/and received by the specified users.
  *