diff --git a/src/Instance.ts b/src/Instance.ts
index 9b99cd5687..a708945644 100644
--- a/src/Instance.ts
+++ b/src/Instance.ts
@@ -273,17 +273,13 @@ export class Marked {
       const origOpt: MarkedOptions = { ...options };
       const opt: MarkedOptions = { ...this.defaults, ...origOpt };
 
-      // Show warning if an extension set async to true but the parse was called with async: false
-      if (isAsyncOptions(this.defaults) && isSyncOptions(origOpt)) {
-        if (!opt.silent) {
-          console.warn('marked(): The async option was set to true by an extension. The async: false option sent to parse will be ignored.');
-        }
+      const throwError = this.#onError(!!opt.silent, !!opt.async);
 
-        opt.async = true;
+      if (isAsyncOptions(this.defaults) && isSyncOptions(origOpt)) {
+        // Throw an error if an extension set async to true but the parse was called with async: false
+        return throwError(new Error('marked(): The async option was set to true by an extension. The async: false option sent to parse will be ignored.'));
       }
 
-      const throwError = this.#onError(!!opt.silent, !!opt.async);
-
       // throw error in case of non string input
       if (typeof src === 'undefined' || src === null) {
         return throwError(new Error('marked(): input parameter is undefined or null'));
diff --git a/src/MarkedOptions.ts b/src/MarkedOptions.ts
index d1b73c779b..8ad955d71e 100644
--- a/src/MarkedOptions.ts
+++ b/src/MarkedOptions.ts
@@ -162,9 +162,9 @@ export interface MarkedAsyncOptions extends _MarkedOptions {
 export type MarkedOptions = MarkedSyncOptions | MarkedAsyncOptions;
 
 export function isAsyncOptions(options: MarkedOptions): options is MarkedAsyncOptions {
-  return 'async' in options && options.async === true;
+  return options.async === true;
 }
 
 export function isSyncOptions(options: MarkedOptions): options is MarkedSyncOptions {
-  return !isAsyncOptions(options);
+  return options.async === false;
 }
diff --git a/test/types/marked.ts b/test/types/marked.ts
index de2bd4bdd2..414b82d4d7 100644
--- a/test/types/marked.ts
+++ b/test/types/marked.ts
@@ -261,13 +261,25 @@ marked.use(asyncExtension);
 const md = '# foobar';
 const asyncMarked: string = await marked(md, { async: true });
 const promiseMarked: Promise<string> = marked(md, { async: true });
-const notAsyncMarked: string = marked(md, { async: false });
+const notAsyncMarked: string = marked(md, { async: false, silent: true });
 const defaultMarked: string = marked(md);
 
 const asyncMarkedParse: string = await marked.parse(md, { async: true });
 const promiseMarkedParse: Promise<string> = marked.parse(md, { async: true });
-const notAsyncMarkedParse: string = marked.parse(md, { async: false });
+const notAsyncMarkedParse: string = marked.parse(md, { async: false, silent: true });
 const defaultMarkedParse: string = marked.parse(md);
+
+try {
+  const notAsyncMarkedThrow: string = marked(md, { async: false, silent: false });
+} catch {
+  console.log('expected throw');
+}
+
+try {
+  const notAsyncMarkedParseThrow: string = marked.parse(md, { async: false, silent: false });
+} catch {
+  console.log('expected throw');
+}
 })();
 
 // Tests for List and ListItem
diff --git a/test/unit/marked.test.js b/test/unit/marked.test.js
index c9c0925a12..9bc1faaccd 100644
--- a/test/unit/marked.test.js
+++ b/test/unit/marked.test.js
@@ -632,7 +632,25 @@ used extension2 walked</p>
     it('should return Promise if async is set by extension', () => {
       marked.use({ async: true });
 
-      assert.ok(marked.parse('test', { async: false }) instanceof Promise);
+      assert.ok(marked.parse('test') instanceof Promise);
+    });
+
+    it('should throw an if async is set by extension and a different async parameter is set', () => {
+      marked.use({ async: true });
+
+      assert.throws(() => marked.parse('test', { async: false }), /The async option was set to true by an extension/);
+    });
+
+    it('should not throw an if async is set by extension and a different async parameter is set and the silent parameter is set', () => {
+      marked.use({ async: true });
+
+      assert.doesNotThrow(() => marked.parse('test', { async: false, silent: true }), /The async option was set to true by an extension/);
+    });
+
+    it('should return a Promise if async is set by extension and a different async parameter is set and the silent parameter is set', () => {
+      marked.use({ async: true });
+
+      assert.ok(marked.parse('test', { async: false, silent: true }) instanceof Promise);
     });
 
     it('should allow deleting/editing tokens', () => {