diff --git a/synapse/storage/databases/main/registration.py b/synapse/storage/databases/main/registration.py
index 16ba545740..c9ce6f05a3 100644
--- a/synapse/storage/databases/main/registration.py
+++ b/synapse/storage/databases/main/registration.py
@@ -939,7 +939,7 @@ class RegistrationStore(RegistrationBackgroundUpdateStore):
         token: str,
         device_id: Optional[str],
         valid_until_ms: Optional[int],
-    ) -> None:
+    ) -> int:
         """Adds an access token for the given user.
 
         Args:
@@ -949,6 +949,8 @@ class RegistrationStore(RegistrationBackgroundUpdateStore):
             valid_until_ms: when the token is valid until. None for no expiry.
         Raises:
             StoreError if there was a problem adding this.
+        Returns:
+            The token ID
         """
         next_id = self._access_tokens_id_gen.get_next()
 
@@ -964,6 +966,8 @@ class RegistrationStore(RegistrationBackgroundUpdateStore):
             desc="add_access_token_to_user",
         )
 
+        return next_id
+
     def _set_device_for_access_token_txn(self, txn, token: str, device_id: str) -> str:
         old_device_id = self.db_pool.simple_select_one_onecol_txn(
             txn, "access_tokens", {"token": token}, "device_id"
diff --git a/synapse/storage/databases/main/schema/delta/58/19txn_id.sql b/synapse/storage/databases/main/schema/delta/58/19txn_id.sql
index 31e81314b4..82c00dd908 100644
--- a/synapse/storage/databases/main/schema/delta/58/19txn_id.sql
+++ b/synapse/storage/databases/main/schema/delta/58/19txn_id.sql
@@ -17,16 +17,23 @@
 -- A map of recent events persisted with transaction IDs. Used to deduplicate
 -- send event requests with the same transaction ID.
 --
--- Note, transaction IDs are scoped to the user ID/access token that was used to
--- make the request.
-CREATE TABLE event_txn_id (
+-- Note: transaction IDs are scoped to the room ID/user ID/access token that was
+-- used to make the request.
+--
+-- Note: The foreign key constraints are ON DELETE CASCADE, as if we delete the
+-- events or access token we don't want to try and de-duplicate the event.
+CREATE TABLE IF NOT EXISTS event_txn_id (
     event_id TEXT NOT NULL,
     user_id TEXT NOT NULL,
     token_id BIGINT NOT NULL,
     txn_id TEXT NOT NULL,
-    inserted_ts BIGINT NOT NULL
+    inserted_ts BIGINT NOT NULL,
+    FOREIGN KEY (event_id)
+        REFERENCES events (event_id) ON DELETE CASCADE,
+    FOREIGN KEY (token_id)
+        REFERENCES access_tokens (id) ON DELETE CASCADE
 );
 
-CREATE UNIQUE INDEX event_txn_id_event_id ON event_txn_id(event_id);
-CREATE UNIQUE INDEX event_txn_id_txn_id ON event_txn_id(user_id, token_id, txn_id);
-CREATE INDEX event_txn_id_ts ON event_txn_id(inserted_ts);
+CREATE UNIQUE INDEX IF NOT EXISTS event_txn_id_event_id ON event_txn_id(event_id);
+CREATE UNIQUE INDEX IF NOT EXISTS event_txn_id_txn_id ON event_txn_id(user_id, token_id, txn_id);
+CREATE INDEX IF NOT EXISTS event_txn_id_ts ON event_txn_id(inserted_ts);
diff --git a/tests/handlers/test_message.py b/tests/handlers/test_message.py
index dbad7dc158..d32de57515 100644
--- a/tests/handlers/test_message.py
+++ b/tests/handlers/test_message.py
@@ -44,12 +44,15 @@ class EventCreationTestCase(unittest.HomeserverTestCase):
         access_token = self.login("tester", "foobar")
         room_id = self.helper.create_room_as(user_id, tok=access_token)
 
-        # We make the IDs up here, which is fine.
-        token_id = 4957834
-        txn_id = "something_suitably_random"
+        info = self.get_success(
+            self.hs.get_datastore().get_user_by_access_token(access_token,)
+        )
+        token_id = info["token_id"]
 
         requester = create_requester(user_id, access_token_id=token_id)
 
+        txn_id = "something_suitably_random"
+
         def create_duplicate_event():
             return self.get_success(
                 handler.create_event(
diff --git a/tests/unittest.py b/tests/unittest.py
index 5c87f6097e..6c1661c92c 100644
--- a/tests/unittest.py
+++ b/tests/unittest.py
@@ -254,17 +254,24 @@ class HomeserverTestCase(TestCase):
         if hasattr(self, "user_id"):
             if self.hijack_auth:
 
+                # We need a valid token ID to satisfy foreign key constraints.
+                token_id = self.get_success(
+                    self.hs.get_datastore().add_access_token_to_user(
+                        self.helper.auth_user_id, "some_fake_token", None, None,
+                    )
+                )
+
                 async def get_user_by_access_token(token=None, allow_guest=False):
                     return {
                         "user": UserID.from_string(self.helper.auth_user_id),
-                        "token_id": 1,
+                        "token_id": token_id,
                         "is_guest": False,
                     }
 
                 async def get_user_by_req(request, allow_guest=False, rights="access"):
                     return create_requester(
                         UserID.from_string(self.helper.auth_user_id),
-                        1,
+                        token_id,
                         False,
                         False,
                         None,