The most common side effects of azithromycin are gastrointestinal: diarrhea, nausea, vomiting, and abdominal pain. Diarrhea is by far the most frequent, affecting roughly 4 in 10 people in some studies. Most of these side effects are mild and resolve on their own once you finish the course, but azithromycin also carries a few rarer risks worth knowing about.
Digestive Side Effects Are the Most Frequent
Stomach and intestinal problems account for the vast majority of complaints. In a retrospective study of pediatric patients published in the Journal of Hospital Pharmacy and Health Services, diarrhea occurred in about 40% of users, vomiting in roughly 10%, and abdominal pain in about 8%. Adult rates tend to be somewhat lower, but diarrhea and nausea consistently top the list across age groups.
These effects happen because azithromycin doesn’t just fight bacteria. It also mimics a gut hormone called motilin, which triggers the muscles lining your stomach and intestines to contract. That’s the same mechanism doctors sometimes exploit on purpose to help patients whose stomachs empty too slowly. But when you’re taking the drug for an infection, those extra contractions translate to cramping, loose stools, and nausea.
If your stomach is sensitive, taking azithromycin with food can help. The tablets absorb well either way, so eating beforehand won’t make the antibiotic less effective. Staying hydrated is especially important if diarrhea is significant. Most digestive symptoms improve within a day or two of finishing treatment.
Heart Rhythm Changes in Higher-Risk Groups
The FDA issued a safety communication warning that azithromycin can cause abnormal heart rhythms, specifically a change in the heart’s electrical timing called QT prolongation. In rare cases, this can lead to a dangerous rhythm called torsades de pointes, which can be fatal.
For most people, this risk is extremely small. Among otherwise healthy patients, the estimated excess risk of cardiovascular death compared to another common antibiotic (amoxicillin) is roughly 1 in 111,000. But for people who already have heart disease or other risk factors, that number climbs to about 1 in 4,100. The groups most vulnerable include people with a history of irregular heartbeats, those with low potassium or magnesium levels, people with a slower than normal heart rate, and anyone already taking medications that affect heart rhythm. Elderly patients and those with existing cardiac conditions are also more susceptible.
Liver Problems Are Rare but Can Appear Quickly
Azithromycin-related liver injury has been reported infrequently, but when it does occur, symptoms typically appear within 3 to 10 days of starting the medication. The damage appears to be a direct effect of the drug rather than an allergic reaction, since it can develop within hours in some cases. Signs to watch for include yellowing of the skin or eyes, unusually dark urine, persistent nausea, or pain in the upper right side of your abdomen. These symptoms are uncommon, but they warrant prompt medical attention.
Skin Reactions
Mild rashes can occur with azithromycin, as with most antibiotics. Far more rarely, severe skin reactions involving widespread rash, fever, and swollen lymph nodes have been reported. These serious reactions, which can also involve internal organs, occur at a rate estimated at more than 1 per 10,000 medication exposures across all drugs that cause them. A rash that spreads rapidly, blisters, or comes with fever is not something to wait out.
Risk of Secondary Gut Infections
Any antibiotic can disrupt the balance of bacteria in your gut and open the door for an infection called C. diff, which causes severe, watery diarrhea that persists or worsens after you finish your antibiotic course. The good news is that azithromycin is one of the lower-risk antibiotics for this. A large U.S. study covering 2008 to 2020 found a C. diff rate of less than 1 case per 10,000 azithromycin prescriptions, placing it in the same low-risk category as clarithromycin and penicillin. If your diarrhea gets worse after you’ve completed treatment rather than better, that’s worth reporting to your doctor, but the odds are in your favor.
A Note for Infants
Azithromycin is sometimes prescribed to very young babies, particularly to prevent whooping cough. Research published through the New England Journal of Medicine found that early exposure to azithromycin, especially during the first two weeks of life, significantly increased the risk of a condition called pyloric stenosis. This is a narrowing of the passage between the stomach and small intestine that causes forceful vomiting and typically requires a minor surgical procedure to correct. The risk is specific to newborns and doesn’t apply to older children or adults.