Get emergency medical help if you have any of these signs of an allergic reaction: hives; difficulty breathing; swelling of your face, lips, tongue, or throat. Call your doctor at once if you have a serious side effect such as: diarrhea that is watery or bloody; chest pain, uneven heartbeats; nausea, stomach pain, low fever, loss of appetite, dark urine, clay-colored stools, jaundice (yellowing of the skin or eyes); or fever, sore throat, and headache with a severe blistering, peeling, and red skin rash.

Less serious side effects may include: mild nausea, vomiting, diarrhea, constipation, or stomach pain; dizziness, tired feeling, or headache; nervous feeling, sleep problems (insomnia); vaginal itching or discharge; mild itching or skin rash; ringing in your ears, problems with hearing; or decreased sense of taste or smell. This is not a complete list of side effects and others may occur. Tell your doctor about any unusual or bothersome side effect.

In clinical trials, most of the reported side effects were mild to moderate in severity and were reversible upon discontinuation of the drug. Potentially serious side effects of angioedema and cholestatic jaundice were reported rarely. Approximately 0.7% of the patients (adults and pediatric patients) from the 5-day multiple-dose clinical trials discontinued azithromycin therapy because of treatment-related side effects. In adults given 500 mg/day for 3 days, the discontinuation rate due to treatment-related side effects was 0.6%. In clinical trials in pediatric patients given 30 mg/kg, either as a single dose or over 3 days, discontinuation from the trials due to treatment-related side effects was approximately 1%.

Many drugs can interact with Zithromax. Below is just a partial list. Tell your doctor if you are using:

digoxin (Lanoxin, Lanoxicaps); carbamazepine (Carbatrol, Tegretol); cyclosporine (Neoral, Sandimmune); phenytoin (Dilantin); pimozide (Orap); theophylline (Theo-Dur, Theolair, Theochron); warfarin (Coumadin); another antibiotic, especially clarithromycin (Biaxin) or erythromycin (E-Mycin, E.E.S, Ery-Tab); a calcium channel blocker such as diltiazem (Cartia XT, Diltiazem, Tiazac), felodipine (Plendil), nicardipine (Cardene), nifedipine (Procardia, Adalat), nimodipine (Nimotop), verapamil (Calan, Covera-HS); cholesterol-lowering medicines such as lovastatin (Mevacor), atorvastatin (Lipitor), or simvastatin (Zocor); ergot medicine such as methysergide (Sansert), ergotamine (Ergostat, Medihaler, Cafergot, Ercaf, Wigraine), dihydroergotamine mesylate (D.H.E., Migranal Nasal Spray); or HIV medicines such as indinavir (Crixivan), nelfinavir (Viracept), ritonavir (Norvir), saquinavir (Invirase).