diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index 91d54ca..3bb631f 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -1915,6 +1915,9 @@
  *
  * This setting requires compiling with clang -fsanitize=memory.
  *
+ * \warning This macro is only used for extended testing; it is not considered
+ * part of the library's API, so it may change or disappear at any time.
+ *
  * Uncomment to enable testing of the constant-flow nature of selected code.
  */
 //#define MBEDTLS_TEST_CONSTANT_FLOW_MEMSAN