diff --git a/packages/core/src/integrations/functiontostring.ts b/packages/core/src/integrations/functiontostring.ts
index 7a50aec67f2d..0f3e9f08b59e 100644
--- a/packages/core/src/integrations/functiontostring.ts
+++ b/packages/core/src/integrations/functiontostring.ts
@@ -1,11 +1,14 @@
-import type { Integration, IntegrationClass, IntegrationFn, WrappedFunction } from '@sentry/types';
+import type { Client, Integration, IntegrationClass, IntegrationFn, WrappedFunction } from '@sentry/types';
 import { getOriginalFunction } from '@sentry/utils';
+import { getClient } from '../exports';
 import { convertIntegrationFnToClass, defineIntegration } from '../integration';
 
 let originalFunctionToString: () => void;
 
 const INTEGRATION_NAME = 'FunctionToString';
 
+const SETUP_CLIENTS = new WeakMap<Client, boolean>();
+
 const _functionToStringIntegration = (() => {
   return {
     name: INTEGRATION_NAME,
@@ -18,20 +21,37 @@ const _functionToStringIntegration = (() => {
       try {
         // eslint-disable-next-line @typescript-eslint/no-explicit-any
         Function.prototype.toString = function (this: WrappedFunction, ...args: any[]): string {
-          const context = getOriginalFunction(this) || this;
+          const originalFunction = getOriginalFunction(this);
+          const context =
+            SETUP_CLIENTS.has(getClient() as Client) && originalFunction !== undefined ? originalFunction : this;
           return originalFunctionToString.apply(context, args);
         };
       } catch {
         // ignore errors here, just don't patch this
       }
     },
+    setup(client) {
+      SETUP_CLIENTS.set(client, true);
+    },
   };
 }) satisfies IntegrationFn;
 
+/**
+ * Patch toString calls to return proper name for wrapped functions.
+ *
+ * ```js
+ * Sentry.init({
+ *   integrations: [
+ *     functionToStringIntegration(),
+ *   ],
+ * });
+ * ```
+ */
 export const functionToStringIntegration = defineIntegration(_functionToStringIntegration);
 
 /**
  * Patch toString calls to return proper name for wrapped functions.
+ *
  * @deprecated Use `functionToStringIntegration()` instead.
  */
 // eslint-disable-next-line deprecation/deprecation
@@ -39,3 +59,6 @@ export const FunctionToString = convertIntegrationFnToClass(
   INTEGRATION_NAME,
   functionToStringIntegration,
 ) as IntegrationClass<Integration & { setupOnce: () => void }>;
+
+// eslint-disable-next-line deprecation/deprecation
+export type FunctionToString = typeof FunctionToString;
diff --git a/packages/core/test/lib/integrations/functiontostring.test.ts b/packages/core/test/lib/integrations/functiontostring.test.ts
index bb3b62d11915..c0e2a22cd6ed 100644
--- a/packages/core/test/lib/integrations/functiontostring.test.ts
+++ b/packages/core/test/lib/integrations/functiontostring.test.ts
@@ -1,7 +1,18 @@
-import { fill } from '../../../../utils/src/object';
-import { FunctionToString } from '../../../src/integrations/functiontostring';
+import { fill } from '@sentry/utils';
+import { getClient, getCurrentScope, setCurrentClient } from '../../../src';
+import { functionToStringIntegration } from '../../../src/integrations/functiontostring';
+import { TestClient, getDefaultTestClientOptions } from '../../mocks/client';
 
 describe('FunctionToString', () => {
+  beforeEach(() => {
+    const testClient = new TestClient(getDefaultTestClientOptions({}));
+    setCurrentClient(testClient);
+  });
+
+  afterAll(() => {
+    getCurrentScope().setClient(undefined);
+  });
+
   it('it works as expected', () => {
     const foo = {
       bar(wat: boolean): boolean {
@@ -17,10 +28,31 @@ describe('FunctionToString', () => {
 
     expect(foo.bar.toString()).not.toBe(originalFunction);
 
-    // eslint-disable-next-line deprecation/deprecation
-    const fts = new FunctionToString();
-    fts.setupOnce();
+    const fts = functionToStringIntegration();
+    getClient()?.addIntegration?.(fts);
 
     expect(foo.bar.toString()).toBe(originalFunction);
   });
+
+  it('does not activate when client is not active', () => {
+    const foo = {
+      bar(wat: boolean): boolean {
+        return wat;
+      },
+    };
+    const originalFunction = foo.bar.toString();
+    fill(foo, 'bar', function wat(whatever: boolean): () => void {
+      return function watwat(): boolean {
+        return whatever;
+      };
+    });
+
+    expect(foo.bar.toString()).not.toBe(originalFunction);
+
+    const testClient = new TestClient(getDefaultTestClientOptions({}));
+    const fts = functionToStringIntegration();
+    testClient.addIntegration(fts);
+
+    expect(foo.bar.toString()).not.toBe(originalFunction);
+  });
 });