The table below lists the undesirable effects reported in clinical trials, and based on post-marketing experience.

In a meta-analysis of 45 randomized controlled trials reviewing 3–5 days of azithromycin compared to other antibiotics that are typically given in longer courses for the treatment of upper respiratory tract infections:

• Azithromycin was discontinued because of adverse events in 0.8% (37 of 4,870) patients

In pooled data from 43 comparative clinical trials in children aged 6 months to 16 years of azithromycin in the treatment of upper and lower respiratory tract infections, skin and soft-tissue infections, and trachoma:

• 1.3% of the 2,655 patients receiving azithromycin withdrew from therapy
• Gastrointestinal events predominated, followed by skin and soft-tissue and central nervous system events
• The main gastrointestinal events reported with azithromycin included diarrhea, loose stools, vomiting, nausea, and abdominal pain

Antacids: In a pharmacokinetic study investigating the effects of simultaneous administration of antacid with azithromycin, no effect on overall bioavailability was seen, although peak serum concentrations were reduced by approximately 24%. In patients receiving both azithromycin and antacids, the drugs should not be taken simultaneously.

Cetirizine: In healthy volunteers, co-administration of a 5-day regimen of azithromycin with 20 mg cetirizine at steady-state resulted in no pharmacokinetic interaction and no significant changes in the QT interval.

Didanosine (dideoxyinosine): Co-administration of 1,200 mg/day azithromycin with 400 mg/day didanosine in six HIV-positive subjects did not appear to affect the steady-state pharmacokinetics of didanosine as compared to placebo.

Digoxin and colchicine: Concomitant administration of macrolide antibiotics, including azithromycin, with P-glycoprotein substrates such as digoxin and colchicine, has been reported to result in increased serum levels of the P-glycoprotein substrate. Therefore, if azithromycin and P-glycoprotein substrates such as digoxin are administered concomitantly, the possibility of elevated serum digoxin concentrations should be considered. Clinical monitoring, and possibly serum digoxin levels, during treatment with azithromycin and after its discontinuation are necessary.

Zidovudine: Single 1,000 mg doses and multiple 1,200 mg or 600 mg doses of azithromycin had little effect on the plasma pharmacokinetics or urinary excretion of zidovudine or its glucuronide metabolite. However, administration of azithromycin increased the concentrations of phosphorylated zidovudine, the clinically active metabolite, in peripheral blood mononuclear cells. The clinical significance of this finding is unclear, but it may be of benefit to patients.

Hepatic cytochrome P450 system: Azithromycin does not interact significantly with the hepatic cytochrome P450 system. It is not believed to undergo the pharmacokinetic drug interactions as seen with erythromycin and other macrolides. Hepatic cytochrome P450 induction or inactivation via cytochrome-metabolite complex does not occur with azithromycin.

Ergot: There is a theoretical possibility of interaction between azithromycin and ergot derivates.

Atorvastatin: Co-administration of atorvastatin (10 mg daily) and azithromycin (500 mg daily) did not alter the plasma concentrations of atorvastatin (based on a HMG CoA-reductase inhibition assay. However, post-marketing cases of rhabdomyolysis in patients receiving azithromycin with statins have been reported).

Carbamazepine: In a pharmacokinetic interaction study in healthy volunteers, no significant effect was observed on the plasma levels of carbamazepine or its active metabolite in patients receiving concomitant azithromycin.

Cimetidine: In a pharmacokinetic study investigating the effects of a single dose of cimetidine, given 2 hours before azithromycin, on the pharmacokinetics of azithromycin, no alteration of azithromycin pharmacokinetics was seen.

Coumarin-type oral anticoagulants: In a pharmacokinetic interaction study, azithromycin did not alter the anticoagulant effect of a single dose of 15 mg warfarin administered to healthy volunteers. There have been reports received in the post-marketing period of potentiated anticoagulation subsequent to co-administration of azithromycin and coumarin-type oral anticoagulants. Although a causal relationship has not been established, consideration should be given to the frequency of monitoring prothrombin time when azithromycin is used in patients receiving coumarin-type oral anticoagulants.

Ciclosporin: In a pharmacokinetic study with healthy volunteers who were administered a 500 mg/day oral dose of azithromycin for 3 days and were then administered a single 10 mg/kg oral dose of ciclosporin, the resulting ciclosporin C_max and AUC_0–5 were found to be significantly elevated (by 24% and 21%, respectively); however, no significant changes were seen in AUC_0–∞. Consequently, caution should be exercised before considering concurrent administration of these drugs. If co-administration of these drugs is necessary, ciclosporin levels should be monitored and the dose adjusted accordingly.

Efavirenz: Co-administration of a single dose of 600 mg azithromycin and 400 mg efavirenz daily for 7 days did not result in any clinically significant pharmacokinetic interactions.

Fluconazole: Co-administration of a single dose of 1,200 mg azithromycin did not alter the pharmacokinetics of a single dose of 800 mg fluconazole. Total exposure and half-life of azithromycin were unchanged by the co-administration of fluconazole; however, a clinically insignificant decrease in C^_max (18%) of azithromycin was observed.

Indinavir: Co-administration of a single dose of 1,200 mg azithromycin had no statistically significant effect on the pharmacokinetics of indinavir administered as 800 mg three times daily for 5 days.

Methylprednisolone: In a pharmacokinetic interaction study in healthy volunteers, azithromycin had no significant effect on the pharmacokinetics of methylprednisolone.

Midazolam: In healthy volunteers, co-administration of 500 mg/day azithromycin for 3 days did not cause clinically significant changes in the pharmacokinetics and pharmacodynamics of a single dose of 15 mg midazolam.

Nelfinavir: Co-administration of azithromycin (1,200 mg) and nelfinavir at steady-state (750 mg three times daily) resulted in increased azithromycin concentrations. No clinically significant adverse effects were observed and no dose adjustment was required.

Rifabutin: Co-administration of azithromycin and rifabutin did not affect the serum concentrations of either drug. Neutropenia was observed in subjects receiving concomitant treatment of azithromycin and rifabutin. Although neutropenia has been associated with the use of rifabutin, a causal relationship to combination with azithromycin has not been established.

Sildenafil: In normal healthy male volunteers, there was no evidence of an effect of azithromycin (500 mg daily for 3 days) on the AUC and C_max of sildenafil or its major circulating metabolite.

Terfenadine: Pharmacokinetic studies have reported no evidence of an interaction between azithromycin and terfenadine. There have been rare cases reported where the possibility of such an interaction could not be entirely excluded; however, there was no specific evidence that such an interaction had occurred.

Theophylline: There is no evidence of a clinically significant pharmacokinetic interaction when azithromycin and theophylline are co-administered to healthy volunteers.

Triazolam: In 14 healthy volunteers, co-administration of 500 mg azithromycin on Day 1 and 250 mg on Day 2 with 0.125 mg triazolam on Day 2 had no significant effect on any of the pharmacokinetic variables for triazolam compared to triazolam and placebo.

Trimethoprim/sulfamethoxazole: Co-administration of trimethoprim/sulfamethoxazole DS (160 mg/800 mg) for 7 days with 1,200 mg azithromycin on Day 7 had no significant effect on peak concentrations, total exposure, or urinary excretion of either trimethoprim or sulfamethoxazole. Azithromycin serum concentrations were similar to those seen in other studies.

Prolongation of the QT interval: Caution is required when using azithromycin in patients with congenital or documented QT prolongation, those receiving treatment with other active substances known to prolong QT interval such as antiarrhythmics of Classes Ia and III, antipsychotic agents, antidepressants, and fluoroquinolones, with electrolyte disturbance, particularly in case of hypokalemia and hypomagnesemia, and with clinically relevant bradycardia, cardiac arrhythmia or severe cardiac insufficiency.¹

ZITHROMAX use is contraindicated in patients with a known hypersensitivity to azithromycin, erythromycin, any macrolide or ketolide antibiotic, or to any of the product excipients.¹ 

• ZITHROMAX is well tolerated with a low incidence of side effects. Undesirable effects reported in clinical trials comprised of Blood and Lymphatic System Disorders, Ear and Labyrinth Disorders, Gastrointestinal Disorders, Hepatobiliary Disorders, Skin and Subcutaneous Tissue Disorders, and General Disorders and Administration Site Conditions¹

• As with erythromycin and other macrolides, rare serious allergic  reactions, including angioedema and anaphylaxis (rarely fatal), Dermatologic reactions including Acute Generalized Exanthematous Pustulosis (AGEP), Stevens-Johnson Syndrome (SJS), Toxic Epidermal Necrolysis (TEN) (rarely fatal), and Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS) have been reported. Some of these reactions with azithromycin have resulted in recurrent symptoms and required a longer period of observation and treatment. If an allergic reaction occurs, the drug should be discontinued and appropriate therapy should be instituted. Physicians should be aware that reappearance of the allergic symptoms may occur when symptomatic therapy is discontinued¹

• Since liver is the principal route of elimination for ZITHROMAX, the use of ZITHROMAX should be undertaken with caution in patients with significant hepatic disease.¹
• Abnormal liver function, hepatitis, cholestatic jaundice, hepatic necrosis, and hepatic failure have been reported, some of which have resulted in death. Discontinue ZITHROMAX immediately if signs and symptoms of hepatitis occur.¹

• Prolonged cardiac repolarization and QT interval, imparting a risk of developing cardiac arrhythmia and torsades de pointes, have been seen in treatment with macrolides, including ZITHROMAX. Prescribers should consider the risk of QT prolongation, which can be fatal when weighing the risks and benefits of ZITHROMAX for at-risk groups including:¹
  • With congenital or documented QT prolongation
  • Currently receiving treatment with other active substances known to prolong QT interval, such as antiarrhythmics of Class IA and III, antipsychotic agents, antidepressants, and fluoroquinolones
  • With electrolyte disturbance, particularly in case of hypokalemia and hypomagnesemia
  • With clinically relevant bradycardia, cardiac arrhythmia or severe cardiac insufficiency
  • Elderly patients may be more susceptible to drug-associated effects on the QT interval

• As with any antibiotic preparation, observation for signs of superinfection with non-susceptible organisms including fungi is recommended¹
• The prevalence of acquired resistance to azithromycin may vary geographically and with time for selected species and local information on resistance is desirable, particularly when treating severe infections. Expert advice should be sought when the local prevalence of resistance is such that the utility of azithromycin in at least some types of infections is questionable¹

• Clostridium difficile associated diarrhea has been reported with use of nearly all antibacterial agents, including azithromycin, and may range in severity from mild diarrhea to fatal colitis¹

• Penicillin is usually the first choice for treatment of pharyngitis/tonsillitis due to Streptococcus pyogenes and also for prophylaxis of acute rheumatic fever. ZITHROMAX is in general effective against streptococcus in the oropharynx, but no data are available that demonstrate the efficacy of azithromycin in preventing rheumatic fever¹

• In patients with severe renal impairment (GFR <10 mL/min), a 33% increase in systemic exposure to ZITHROMAX was observed.¹

• Due to the sucrose content in the ZITHROMAX powder for oral suspension (3.87 g / 5 mL of reconstituted suspension), caution should be exercised in diabetic patients. ZITHROMAX should not be taken by patients with rare hereditary problems of fructose intolerance, glucose-galactose malabsorption, or sucrase-isomaltase insufficiency¹

• While most studies do not suggest an association with adverse fetal effects such as major congenital malformations or cardiovascular malformations, there is limited epidemiological evidence of an increased risk of miscarriage following ZITHROMAX exposure in early pregnancy¹
• ZITHROMAX should only be used during pregnancy if clinically needed and the benefit of treatment is expected to outweigh any small increased risk which may exist¹

• Limited information available from published literature indicates that ZITHROMAX is present in human milk at an estimated highest median daily dose of 0.1 to 0.7 mg/kg/day. No serious adverse effects of ZITHROMAX on the breast-fed infants were observed¹
• A decision must be made whether to discontinue breast-feeding or to discontinue/abstain from ZITHROMAX therapy taking into account the benefit of breast-feeding for the child and the benefit of therapy for the woman¹