Harvoni may potentially cause side effects ranging from mild to severe. The list below includes some of the most common side effects but is not a complete list. Mild side effects may go away after a few days or weeks of consistent medication-taking, but if they persist or worsen you should speak with your doctor. Common side effects: Fatigue Dizziness Headache Nausea Diarrhea Muscle Aches Severe side effects: Hepatitis B reactivation (in people coinfected with hep B and hep C) Suicidal Thoughts In some rare cases, patients can experience allergic reactions. Seek emergency medical care if you experience any of the following symptoms after taking Harvoni. Skin rash Wheezing Tightness in the throat or chest Difficulty talking or breathing Swelling of the face, lips, tongue, or throat