diff --git a/packages/react/src/lib/contentEditableComposer.js b/packages/react/src/lib/contentEditableComposer.js
index ffa3f6f9c..03e568aa1 100644
--- a/packages/react/src/lib/contentEditableComposer.js
+++ b/packages/react/src/lib/contentEditableComposer.js
@@ -44,6 +44,30 @@ const findLink = (text, start) => {
   };
 };
 
+const isAlphanumeric = (ch) => Boolean(ch && /[a-zA-Z0-9]/.test(ch));
+const isWhitespace = (ch) => Boolean(ch && /\s/.test(ch));
+
+const isValidEmphasis = (marker, text, start, end) => {
+  if (marker === '`' || marker === '~~') {
+    return true;
+  }
+  const prevChar = start > 0 ? text[start - 1] : '';
+  const firstInside = text[start + marker.length];
+  const lastInside = text[end - 1];
+  const nextChar =
+    end + marker.length < text.length ? text[end + marker.length] : '';
+
+  if (isWhitespace(firstInside) || isWhitespace(lastInside)) return false;
+
+  // CommonMark: only underscore delimiters cannot be intra-word
+  if (marker === '_' || marker === '__') {
+    if (isAlphanumeric(prevChar)) return false;
+    if (isAlphanumeric(nextChar)) return false;
+  }
+
+  return true;
+};
+
 const appendMarkdown = (parent, text) => {
   const tokens = [
     ['**', 'strong'],
@@ -91,7 +115,10 @@ const appendMarkdown = (parent, text) => {
 
       if (token) {
         const [marker, tagName] = token;
-        const end = text.indexOf(marker, index + marker.length);
+        let end = text.indexOf(marker, index + marker.length);
+        while (end !== -1 && !isValidEmphasis(marker, text, index, end)) {
+          end = text.indexOf(marker, end + marker.length);
+        }
         if (end > index + marker.length) {
           flush();
           const formatted = document.createElement(tagName);