The Z-Pack (azithromycin) is a five-day antibiotic prescribed for mild to moderate bacterial infections, most commonly sinus infections, bronchitis flare-ups, and certain types of pneumonia. It comes as a pack of six pills: 500 mg on the first day, then 250 mg daily for four more days. Despite its popularity, it only works against bacteria, not the viruses behind most colds and cases of the flu.
Respiratory Infections
The most common reason doctors prescribe a Z-Pack is for bacterial infections of the airways and sinuses. Specifically, it’s approved for acute bacterial sinusitis, bacterial flare-ups of chronic bronchitis, and community-acquired pneumonia (the kind you catch outside a hospital). It targets several of the bacteria most often responsible for these infections, including the ones behind “walking pneumonia,” which tends to cause a persistent dry cough and milder symptoms than typical pneumonia.
For sinus infections and bronchitis flare-ups, a Z-Pack is usually a second-choice option rather than a first-line treatment. Doctors often prefer amoxicillin for bacterial sinusitis, for instance. But azithromycin becomes useful when someone is allergic to penicillin-type antibiotics or hasn’t responded to the initial treatment.
Strep Throat and Tonsillitis
A Z-Pack can treat strep throat and tonsillitis, but only as an alternative when the preferred antibiotics aren’t an option. Penicillin and amoxicillin remain the go-to choices for strep because they’re more reliably effective and help prevent complications like rheumatic fever. If you’re allergic to penicillin, azithromycin is one of the antibiotics your doctor may turn to instead. This applies to both adults and children over age two.
Ear Infections in Children
For children older than six months, a Z-Pack (in liquid suspension form) is approved for acute middle ear infections. Ear infections are one of the most frequent reasons kids receive antibiotics, and azithromycin is a convenient option because its shorter course and once-daily dosing can be easier for parents to manage compared to antibiotics that need to be given two or three times a day for ten days.
Skin Infections
Azithromycin is approved for uncomplicated skin and soft tissue infections, the kind caused by common bacteria like staph and strep. These might include infected cuts, mild cellulitis, or similar surface-level infections. For deeper or more serious skin infections, doctors typically choose other antibiotics with stronger coverage.
Sexually Transmitted Infections
A Z-Pack has historically played a significant role in treating certain STIs. A single 1-gram dose of azithromycin has been a standard treatment for chlamydia, and for years it was paired with another antibiotic as dual therapy for gonorrhea.
That landscape has shifted. The CDC no longer recommends azithromycin as part of routine gonorrhea treatment because the bacteria that causes gonorrhea has developed increasing resistance to it. The current recommendation for gonorrhea is a single injection of ceftriaxone. If chlamydia hasn’t been ruled out, doctors now prefer doxycycline over azithromycin as the add-on treatment. Azithromycin is also approved for chancroid, a genital ulcer disease that’s relatively rare in the United States.
Why It Doesn’t Work for Colds or the Flu
One of the biggest misunderstandings about the Z-Pack is that it can help with a bad cold, the flu, or a typical case of bronchitis. The vast majority of upper respiratory infections and acute bronchitis are caused by viruses, and antibiotics have zero effect on viruses. A randomized trial published in The Lancet compared azithromycin to vitamin C for acute bronchitis and found no difference: 89% of patients in both groups had returned to normal activities within a week. The antibiotic offered no advantage at all.
This matters because unnecessary antibiotic use drives resistance. Nearly half of the most common pneumonia-causing bacteria in the United States are already resistant to azithromycin. In some countries, that figure exceeds 80%. Every unnecessary prescription makes this problem worse, which is why doctors are increasingly cautious about prescribing a Z-Pack “just in case.”
How It Works in Your Body
Azithromycin belongs to a class of antibiotics called macrolides. It kills bacteria by blocking their ability to build proteins. Without new proteins, the bacteria can’t grow or reproduce. One reason azithromycin works well as a short five-day course is that it concentrates in your tissues and stays active long after you take the last pill. Drug levels in infected tissue remain high for several days after the course ends, so the antibiotic keeps working even though you’ve stopped taking it.
Common Side Effects
Most people tolerate a Z-Pack well, but digestive symptoms are the most frequent complaint. Nausea, diarrhea, stomach cramps, and vomiting can occur. These are usually mild and resolve once the course is finished.
A more serious but less common concern involves the heart. The FDA has warned that azithromycin can cause abnormal heart rhythms in certain people. Those at higher risk include people with existing heart conditions, abnormally slow heart rates, low potassium or magnesium levels, or anyone already taking medications that affect heart rhythm. Older adults and people with heart failure are also more susceptible. If you have a known heart condition, your doctor should weigh this risk before prescribing it.
Azithromycin is processed by the liver and eliminated through bile, so people with significant liver disease should generally avoid it. In rare cases, it has been linked to severe liver reactions, including symptoms like sudden fatigue, yellowing of the skin or eyes, and dark urine.
What It Treats vs. What It’s Prescribed For
There’s often a gap between what a Z-Pack is approved to treat and how it’s actually used. Studies suggest a large share of azithromycin prescriptions go toward conditions where it’s unlikely to help, particularly viral upper respiratory infections. If your doctor prescribes a Z-Pack, it’s reasonable to ask whether your illness is bacterial or viral. For bacterial infections within its approved range, azithromycin remains effective and convenient. For everything else, the five-day pack sitting in your medicine cabinet won’t speed your recovery.