diff --git a/docs/admin_api/purge_history_api.rst b/docs/admin_api/purge_history_api.rst
index f7be226fd983d8f4646e4f8524395034436b073d..f2c4dc03acdd6da92c22a92a4573c447699d8449 100644
--- a/docs/admin_api/purge_history_api.rst
+++ b/docs/admin_api/purge_history_api.rst
@@ -8,6 +8,9 @@ Depending on the amount of history being purged a call to the API may take
 several minutes or longer. During this period users will not be able to
 paginate further back in the room from the point being purged from.
 
+Note that, in order to not break the room, this API won't delete the last
+message sent to it.
+
 The API is:
 
 ``POST /_synapse/admin/v1/purge_history/<room_id>[/<event_id>]``