Guides
Azithromycin (Zithromax, Z-Pack): uses and when it is not needed
Azithromycin (Zithromax, the "Z-Pack") is a macrolide antibiotic that treats certain bacterial infections, such as some infections of the lungs, sinuses, throat, skin and genital tract. It does not work for colds, flu or other viruses, and most coughs and bronchitis in healthy people are viral and improve without an antibiotic. It can affect the heart's rhythm (QT interval) and cause diarrhea. In a video visit, the doctor evaluates whether an antibiotic is right for you.
Is it an emergency? Call 911.
Pay now, then choose your time right away. Full refund if you cancel 24 hours or more ahead.
See the cancellation policy
You can cancel for a full refund or change the time up to 24 hours before your visit. Within 24 hours you can't cancel and there's no refund. If you don't join in the first 10 minutes, there's no refund.
If you choose a time less than 24 hours away, it can't be cancelled or changed.
If the clinic can't see you, for example because of a technical problem on our side, we refund you in full.
Is this an emergency?
This is not an emergency service. Call 911 or go to the nearest emergency room if you have:
- Trouble breathing, blue lips or very fast breathing
- Swelling of the face, lips or throat, or hives with shortness of breath
- Palpitations, irregular heartbeat, fainting or chest pain
- Bloody diarrhea, dehydration, or high fever with severe abdominal pain
- Yellow skin or eyes, or dark urine
- A baby under 3 months old with a fever
Help lines
What is azithromycin?
Azithromycin is an antibiotic from the macrolide family. According to MedlinePlus, it helps stop the growth of the bacteria that cause certain infections. Its brand name is Zithromax, and its short five-day pack is known as the Z-Pack or "Z-Pak". It requires a prescription.
One distinctive feature is that azithromycin stays in the body's tissues for several days, which is why many courses with it are short. That does not make it "stronger" than other antibiotics; it just means it is used differently. Each infection has its own antibiotic and length of treatment, which the doctor chooses based on your case.
What is azithromycin used for?
According to MedlinePlus and the FDA label published on DailyMed, azithromycin is indicated for infections caused by bacteria that are sensitive to it, including:
- Lung infections: mild community-acquired pneumonia and bacterial flare-ups of chronic obstructive pulmonary disease (COPD).
- Acute bacterial sinusitis. See sinus infection.
- Throat and tonsil infections (pharyngitis and tonsillitis), as an alternative when the first-line antibiotic cannot be used, for example because of a penicillin allergy. See sore throat.
- Uncomplicated skin infections.
- Sexually transmitted infections such as urethritis and cervicitis caused by chlamydia. See chlamydia.
- Whooping cough. The CDC lists macrolides among the preferred antibiotics, and azithromycin is the choice for babies under one month. See whooping cough.
- Other infections, such as those caused by *Mycobacterium avium* complex in people with HIV, which are managed with specialists.
What azithromycin does NOT treat: colds, flu and most bronchitis
The most common search is "I have a cough and want the five-day pill". Here is the key point: antibiotics do not work against viruses. MedlinePlus says it directly: azithromycin does not help with colds, flu or other viral infections. Taking it when it is not needed does not speed up recovery, and it does expose you to its side effects.
According to the CDC, most cases of acute bronchitis (a "chest cold") in healthy people are caused by viruses and get better on their own; antibiotics generally do not help. The same is true of the common cold and of most sinus infections, which are viral. A cough can last two or three weeks after a virus without that meaning an antibiotic is needed. See cough and bronchitis.
There is another reason not to use it "just in case": the FDA label asks that it be prescribed only for infections that are proven or strongly suspected to be bacterial, to reduce antibiotic resistance. Every antibiotic taken without need makes it more likely to fail when you truly need it.
Signs a cough or respiratory infection may be bacterial
No single sign confirms it, which is why an evaluation is needed. These are situations in which the doctor will want to look more closely, because they can point to a bacterial infection, pneumonia or another cause:
- High fever, fever lasting more than three or four days, or a fever that returns after improving.
- Cold symptoms that improve and then get worse again ("double worsening").
- Sinus symptoms for 10 days or more without improving, or with strong facial pain and fever.
- A cough with shortness of breath, pain on deep breaths or fast breathing.
- A very sore throat with fever and no cough (could be strep, which is evaluated differently).
- A cough lasting more than three weeks, or coughing up blood.
- Lung or heart disease, diabetes, a weakened immune system or older age: in these cases it is better to be evaluated sooner.
Traveler's diarrhea and azithromycin
Another reason people search for azithromycin is traveler's diarrhea. The CDC (Yellow Book) explains that most cases are mild and resolve with fluids, and that antibiotics are reserved for moderate to severe cases or for fever or blood in the stool (dysentery). Among the options, azithromycin is one that is mentioned, partly because fluoroquinolone resistance is common in some parts of the world.
Even so, it should not be packed or taken on your own "just in case". The doctor decides whether it is needed based on your destination, symptoms, allergies and the medicines you take. Staying hydrated is the most important step in any diarrhea. See traveler's diarrhea.
Azithromycin and the heart: the QT interval
The FDA label and a 2013 FDA safety communication warn that azithromycin can alter the heart's electrical activity (QT prolongation). In rare cases this can cause a serious irregular rhythm (torsades de pointes). For most people the risk is low, but it is higher if:
- You already have a long QT interval or a history of irregular rhythms.
- You have low potassium or magnesium (which can happen with severe vomiting or diarrhea or with certain diuretics).
- You have a very slow pulse, heart failure or other heart disease.
- You take heart-rhythm medicines (such as amiodarone, sotalol, quinidine or procainamide) or other drugs that also lengthen the QT.
- You are older.
Azithromycin side effects
The most common effects listed by MedlinePlus and the FDA label are diarrhea, nausea, stomach pain, vomiting and sometimes headache. Most are mild. Tell the doctor if they do not go away.
Get medical help if you have any of these, which are less common but important:
- Serious allergic reaction: hives, swelling of the face or throat, trouble breathing. Severe skin reactions (such as Stevens-Johnson syndrome) are rare.
- Severe, watery or bloody diarrhea during treatment or up to several months after: it can be a *C. difficile* infection, an effect of antibiotics in general.
- Irregular heartbeat, dizziness or fainting.
- Yellow skin or eyes, dark urine or pain in the upper right abdomen: signs of a liver problem.
- Worsening muscle weakness in people with myasthenia gravis.
Interactions: what not to mix with azithromycin
Before starting, tell the doctor and the pharmacist everything you take. According to the FDA label and MedlinePlus, the most important are:
- Medicines that lengthen the QT or treat heart rhythm: they can add to the risk. This includes antiarrhythmics and some antipsychotics and antidepressants, as well as other antibiotics such as fluoroquinolones.
- Warfarin (blood thinner): changes in the blood-thinning effect have been reported, so your blood tests are monitored.
- Antacids with aluminum or magnesium: MedlinePlus asks you to check with your doctor or pharmacist about when to take them, because they can lower the antibiotic level if taken at the same time.
- Other medicines: some cholesterol medicines (statins), cyclosporine and others can interact. Keep a list of everything you take, including supplements.
- Allergies: if you ever had a reaction to azithromycin, erythromycin or clarithromycin, tell the doctor.
Pregnancy, breastfeeding and children
Pregnancy and breastfeeding. If you are pregnant, planning a pregnancy or breastfeeding, tell the doctor: it is a decision made by weighing benefits and risks for you and the baby. Do not self-medicate.
Children and babies. Azithromycin is used in children when indicated, but the dose depends on weight and the infection, and in newborns the label warns of a rare risk of infantile hypertrophic pyloric stenosis. That is why a child with fever or cough should be evaluated before getting any antibiotic. In video visits, patients under 21 are seen with a parent or guardian on camera. See the child visit.
If you are prescribed azithromycin: using it well
This is general information; always follow the instructions on your prescription and from the pharmacy.
- Finish the course as directed, even if you feel better sooner, unless the doctor tells you otherwise or you have a reaction.
- Do not share it or save leftovers for another illness or another person.
- If you miss a dose, take it when you remember unless it is almost time for the next one; do not double up. Ask the pharmacist.
- Stay hydrated. If you have diarrhea, do not take anti-diarrhea medicine without checking first, because in some infections it can make things worse.
- If you are not better in two or three days, or you get worse, tell the doctor: the cause may not be bacterial, or a different plan may be needed.
Book a video evaluation
Book an evaluation; the doctor decides whether a treatment is right for you.
You see Dr. Diana Nieves Castro, MD, a Puerto Rico–licensed general practice physician, in English or Spanish. Every day, any time (Puerto Rico time). Doctor online 24/7: video doctor visits 24/7. The doctor evaluates your cough, cold, sinus symptoms, sore throat or diarrhea, reviews the medicines you already take and your heart history, and explains your options, which often do not include an antibiotic. If you are very short of breath, have chest pain or faint, call 911. See the online doctor visit and read more about antibiotics by telehealth.
The pharmacy bills for any medicine. We do not bill insurance; on request we give you an itemized receipt (superbill). See pricing. Book your visit.
When to get emergency help
If you are taking azithromycin or have a respiratory infection and have any of these signs, call 911 or go to the emergency room. See when to go to the ER.
- Trouble breathing, blue lips or very fast breathing
- Swelling of the face, lips or throat, or hives with shortness of breath (severe allergic reaction)
- Palpitations, a very irregular heartbeat, fainting or chest pain
- Bloody diarrhea, dehydration, or high fever with severe abdominal pain
- Confusion, a stiff neck with fever, or heavy drowsiness
- A baby under 3 months old with a fever
How we care for you
We evaluate, diagnose and treat; if needed, we give you a lab order for a lab near you.
We see adults and children of all ages. Patients under 21: a parent with patria potestad (parental authority) or a legal guardian is on camera for the whole visit and shows photo ID.
Real-time video visit. We send you a secure video link. It works on your phone, tablet or computer.
If appropriate, the doctor prescribes and we send your prescription to your pharmacy.
You must be physically in Puerto Rico during the visit (visitors, tourists and cruise passengers are welcome).
In an emergency, call 911.
Frequently asked questions
What is azithromycin used for?
Does azithromycin work for a cold, the flu or a cough?
What is a Z-Pack?
Can I buy azithromycin over the counter in Puerto Rico?
Does azithromycin affect the heart?
What are the side effects of azithromycin?
Can I take azithromycin with alcohol, antacids or birth control?
Is azithromycin used for traveler's diarrhea?
How long does azithromycin take to work?
Do I have to finish the whole course even if I feel better?
About this page
Author: The Health Initiative
Medical care provided by Dr. Diana Nieves Castro, MD (PR license 24027)
Disclosure: Founding Executive Director of The Health Initiative
Published: 2026-10-10 · Updated: 2026-10-11
Sources (8)
- Azithromycin (MedlinePlus) (opens in a new tab) (accessed 2026-10-10)
- Azitromicina (MedlinePlus) (opens in a new tab) (accessed 2026-10-10)
- ZITHROMAX (azithromycin) FDA label: indications, warnings and interactions (DailyMed, NLM) (opens in a new tab) (accessed 2026-10-10)
- FDA Drug Safety Communication: azithromycin and the risk of potentially fatal heart rhythms, 2013 (opens in a new tab) (accessed 2026-10-10)
- Acute bronchitis (CDC) (opens in a new tab) (accessed 2026-10-10)
- Pertussis: clinical care and treatment (CDC) (opens in a new tab) (accessed 2026-10-10)
- Travelers' Diarrhea, CDC Yellow Book (CDC) (opens in a new tab) (accessed 2026-10-10)
- Chlamydia: 2021 STI treatment guidelines (CDC) (opens in a new tab) (accessed 2026-10-10)