Patients who take the antibiotic Zithromax have reported mild side effects such as an upset stomach, headaches, loose stools or diarrhea, nausea or vomiting. However, if you suffer bloody or watery stools, seizures, a loss of appetite, muscle weakness, chest pain, swelling of the throat, face, mouth, tongue or lips, or tightness in the chest after taking the drug, be sure to talk to your doctor immediately.