Along with its needed effects, azithromycin may cause some unwanted or undesirable effects. Generally, azithromycin is well tolerated and many people will not experience unwanted effects. The frequency and severity of these effects is dependant on many factors including dose, duration of therapy and individual susceptibility. Possible unwanted effects include:

• diarrhea
• nausea
• vomiting
• stomach pain

Uncommon:

• allergy (hives, itching, difficulty breathing)
• vaginal infection

Azithromycin is an antibiotic. It works by inhibiting the growth of susceptible bacteria by blocking vital proteins required by bacteria. Azithromycin slows bacterial growth. Azithromycin does not suppress growth of all types of bacteria, but only those that have a sensitivity to this antibiotic. Infections caused by bacteria that are not sensitive to azithromycin will not show improvement after taking this medication.

Allergic reactions can occur with azithromycin use. People with a history of allergy, asthma, hay fever or hives seem to be more susceptible to these reactions. The reaction can be immediate and severe. Allergic symptoms include wheezing, hives, itching, swelling, spasms in the throat and breathing tubes, joint and muscle pain, difficulty breathing, fever and skin rashes. Nausea and vomiting are not symptoms of an allergic reaction.
Taking the antibiotic repeatedly or for prolonged periods may result in bacterial or fungal overgrowth which can lead to a second infection. When this occurs, the azithromycin may need to be stopped and another antibiotic prescribed to treat the new infection.
Diarrhea often develops while taking azithromycin. This is sometimes caused by an overgrowth of bacteria in the gut that are not killed by the antibiotic. In severe cases, this may be life threatening and would require treatment with other antibiotics. In mild cases, symptoms disappear shortly after the drug is discontinued.
Drug Interactions: It is important to tell your doctor and pharmacist of any prescription or over-the-counter medications you are taking. In some cases, the dose of one or both drugs may need to be altered or another drug may be prescribed.

The following drugs or drug classes have been known to interact with azithromycin:

• antacids containing aluminum or magnesium
• warfarin
• carbamazepine
• cyclosporine
• digoxin
• disopyramide
• theophylline 
• triazolam 
• phenytoin

Use is not recommended in the following situations:

• allergy to azithromycin, erythromycin or any of the macrolide antibiotics

Caution is recommended in the following situations

• liver disease
• kidney disease

Use in pregnancy: Azithromycin does cross the placenta. Consult your doctor or pharmacist if you suspect you may be pregnant.

Use while breastfeeding: Azithromycin passes into the breast-milk although levels are low.  Consult your doctor or pharmacist before you take this medication while breastfeeding.