diff --git a/docs/hazmat/primitives/symmetric-encryption.rst b/docs/hazmat/primitives/symmetric-encryption.rst
index f4d0457..8ed64c7 100644
--- a/docs/hazmat/primitives/symmetric-encryption.rst
+++ b/docs/hazmat/primitives/symmetric-encryption.rst
@@ -352,6 +352,11 @@
                                         Do not reuse an ``initialization_vector``
                                         with a given ``key``.
 
+    .. note::
+
+        Do not truncate the GCM authentication tag unless absolutely necessary.
+        If you must truncate the minimum allowable length is 4 bytes.
+
     :param bytes tag: The tag bytes to verify during decryption. When encrypting
                       this must be None.