diff --git a/README.md b/README.md
index 756b8c71ca9..5272706e432 100644
--- a/README.md
+++ b/README.md
@@ -325,6 +325,8 @@ await matrixClient.initRustCrypto();
 
 After calling `initRustCrypto`, you can obtain a reference to the [`CryptoApi`](https://matrix-org.github.io/matrix-js-sdk/interfaces/crypto_api.CryptoApi.html) interface, which is the main entry point for end-to-end encryption, by calling [`MatrixClient.getCrypto`](https://matrix-org.github.io/matrix-js-sdk/classes/matrix.MatrixClient.html#getCrypto).
 
+**WARNING**: the cryptography stack is not thread-safe. Having multiple `MatrixClient` instances connected to the same Indexed DB will cause data corruption and decryption failures. The application layer is responsible for ensuring that only one `MatrixClient` issue is instantiated at a time.
+
 ## Secret storage
 
 You should normally set up [secret storage](https://spec.matrix.org/v1.12/client-server-api/#secret-storage) before using the end-to-end encryption. To do this, call [`CryptoApi.bootstrapSecretStorage`](https://matrix-org.github.io/matrix-js-sdk/interfaces/crypto_api.CryptoApi.html#bootstrapSecretStorage).
diff --git a/src/client.ts b/src/client.ts
index 478e2784e51..dbe1363942c 100644
--- a/src/client.ts
+++ b/src/client.ts
@@ -2222,6 +2222,10 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
      *
      * An alternative to {@link initCrypto}.
      *
+     * **WARNING**: the cryptography stack is not thread-safe. Having multiple `MatrixClient` instances connected to
+     * the same Indexed DB will cause data corruption and decryption failures. The application layer is responsible for
+     * ensuring that only one `MatrixClient` issue is instantiated at a time.
+     *
      * @param args.useIndexedDB - True to use an indexeddb store, false to use an in-memory store. Defaults to 'true'.
      * @param args.storageKey - A key with which to encrypt the indexeddb store. If provided, it must be exactly
      *    32 bytes of data, and must be the same each time the client is initialised for a given device.