What is the most common side effect of cephalexin?

The most common cephalexin side effects are diarrhea, nausea, and stomach pain. Headache, fatigue, and vaginal yeast infections can also happen. Serious cephalexin side effects are rare, but possible. They include infectious diarrhea, seizures, and an increased risk of bleeding. Severe allergic reactions can also happen.  View original source