Most people who take azithromycin finish their course with nothing worse than a few loose stools. A smaller group has a rough couple of days with nausea or cramping. And a very small number run into problems that need prompt attention: an allergic reaction that keeps coming back, liver irritation, ringing in the ears, or a heart rhythm disturbance.
This guide sorts azithromycin side effects by where you feel them, starting with the stomach (by far the most common), then the ears, then the heart, and finally the less common effects that the official label still asks doctors and patients to watch for. Every percentage below comes from the FDA-approved prescribing information or from published research, and we flag clearly when a number comes from a long-term study rather than a short course.
The Big Picture in Numbers
The Zithromax prescribing information (updated July 2026) summarizes clinical trial experience this way: most side effects were mild to moderate and went away after the drug was stopped. Only about 0.7% of patients in the 5-day trials, and 0.6% of adults on the 3-day schedule, quit because of side effects. Most of those who did stop had digestive complaints.
| Adult regimen | Diarrhea / loose stools | Nausea | Abdominal pain | Vomiting |
|---|---|---|---|---|
| Multiple-dose courses (3-day or 5-day) | 4 to 5% | 3% | 2 to 3% | 1% or less |
| Single 1 gram dose | 7% | 5% | 5% | 2% |
| Single 2 gram dose | 14% | 18% | 7% | 7% |
The lesson in that table is simple: the bigger the dose taken at once, the more likely the stomach is to complain. Standard 3-day and 5-day packs sit at the gentle end. If you are comparing those packs, our guide to the azithromycin 3-day vs 5-day pack goes through the schedules in detail.
Part 1: Stomach and Gut Effects
Why azithromycin upsets the stomach
Macrolide antibiotics, the family azithromycin belongs to, are known to stimulate the gut’s motilin receptors, which speeds up the muscular waves that move food along. That is why cramping, urgency and loose stools are typical. Azithromycin also disturbs normal gut bacteria, as every antibiotic does, which can loosen stools for a few days.
What is normal
- A few loose or softer stools during the course or a day or two after.
- Mild queasiness, often worst an hour or two after a dose.
- Bloating, gas or indigestion.
- A changed or odd taste, which the label lists among postmarketing reports.
What is not normal
- Watery diarrhea several times a day, especially with fever, cramping or blood or mucus in the stool.
- Diarrhea that starts days or even weeks after finishing the course.
- Vomiting so frequent you cannot keep fluids down.
The first two can signal Clostridioides difficile infection, a toxin-producing gut bacterium that can overgrow after antibiotics. The label warns that this can range from mild diarrhea to life-threatening colitis and can appear more than two months after antibiotic use. Do not treat it at home with anti-diarrhea medicine before talking to a clinician. Our article on amoxicillin and diarrhea explains the same warning signs for a different antibiotic.
Practical steps that often help
- Take it with food if your stomach is sensitive. The current tablets and suspension can be taken with or without meals, and food raised peak levels slightly without changing total absorption in the label’s studies.
- Keep fluids up. Small sips of water or an oral rehydration drink beat large gulps when nauseated.
- Choose bland meals for a day or two. Rice, toast, bananas and soups are easier than fried or spicy food.
- Space out heartburn remedies. Aluminum or magnesium antacids blunt the peak level of azithromycin; see azithromycin and antacids for timing.
- Skip alcohol until you feel well. It does not react with the drug chemically, but it adds to nausea and dehydration. More in can you drink on azithromycin.
- Ask before using probiotics or anti-diarrheals. Pharmacists can tell you whether either makes sense for your situation.
Part 2: Hearing and Ear Effects
Hearing problems are one of the less discussed azithromycin side effects, partly because they are uncommon with short courses. The label’s postmarketing section lists hearing disturbances including hearing loss, deafness and tinnitus (ringing in the ears), along with changes in taste and smell.
What the long-term study showed
The clearest data come from people taking azithromycin every day for a long time, not from a 5-day pack. In a large trial published in the New England Journal of Medicine in 2011, adults with COPD took 250 mg daily for a year to prevent flare-ups. Audiograms showed hearing decrements in 25% of the azithromycin group compared with 20% on placebo. The researchers suggested their criteria may have overestimated hearing loss in both groups, but the difference was statistically significant, and the trial excluded people who already had hearing impairment.
For the typical person finishing a short pack, hearing changes are rare. The risk climbs with long-term use, higher doses and in people taking other drugs that can affect the ear.
Interaction worth knowing
The label notes that the HIV medicine nelfinavir roughly doubles azithromycin levels and advises close monitoring for known side effects, specifically naming liver enzyme changes and hearing impairment. If you take antiretrovirals, mention them before starting.
What to do if your ears change
- New ringing, muffled hearing, or a sense of fullness that is not from a cold: call your prescriber the same day.
- Sudden hearing loss in one ear: treat as urgent and seek care promptly, whatever the cause, because early evaluation matters.
- Mention any existing hearing loss before long-term azithromycin is started. Clinicians often arrange baseline hearing tests for long courses.
Part 3: Heart Rhythm Effects
This is the side effect that made headlines. In March 2013, the FDA issued a Drug Safety Communication warning that azithromycin can cause abnormal changes in the heart’s electrical activity that may lead to a potentially fatal irregular rhythm.
What the heart effect is
Azithromycin can lengthen the QT interval, a measurement on an ECG that reflects how long the heart’s lower chambers take to recharge between beats. A longer QT increases the chance of a dangerous rhythm called torsades de pointes. In the label’s controlled study, adding azithromycin to chloroquine increased the corrected QT by an average of 5 to 9 milliseconds depending on the dose.
How rare is serious harm?
The label states that observational studies have found roughly double the short-term risk of acute cardiovascular death with azithromycin compared with other antibiotics such as amoxicillin, with five-day cardiovascular death rates ranging from 20 to 400 per million treatment courses. The risk appeared greatest during the first five days and was not limited to people with known heart disease, though the data cannot prove cause and effect.
A widely cited 2012 study estimated 47 extra cardiovascular deaths per million courses compared with amoxicillin, rising to about 245 per million in people with the highest baseline heart risk. For a healthy young adult, that is a very small number. For an older person with heart disease on several rhythm-affecting medicines, it is worth a conversation. Our dedicated article on azithromycin and QT prolongation goes through who should be careful.
Heart symptoms that need immediate care
- Fainting or nearly fainting
- A pounding, racing or irregular heartbeat that feels new
- Sudden dizziness with shortness of breath
- Chest pain
The FDA advises seeking immediate care for an irregular heartbeat, shortness of breath, dizziness or fainting while on azithromycin.
Part 4: Other Effects the Label Asks You to Watch For
Allergic and skin reactions that can return
Serious allergic reactions are rare, but the label describes angioedema (swelling of the face, lips or throat), anaphylaxis, and severe skin reactions such as Stevens-Johnson syndrome, toxic epidermal necrolysis, acute generalized exanthematous pustulosis and DRESS. An unusual detail is that in some people, allergic symptoms came back after treatment of the reaction stopped, even without another dose. The label suggests this may relate to how long azithromycin lingers in tissue, and notes some patients needed prolonged observation.
If you develop hives, swelling, trouble breathing, blistering, peeling skin or a widespread rash with fever, stop taking the drug and get urgent help. Our guide to amoxicillin rash vs allergy describes the difference between harmless rashes and dangerous ones, and the same principles apply here.
Liver irritation
The label reports abnormal liver tests, hepatitis, cholestatic jaundice and, rarely, liver failure. Warning signs include yellowing of the skin or eyes, dark urine, pale stools, itching, right upper belly pain and unusual fatigue. Stop the drug and call your prescriber if these appear. People who previously had jaundice or liver problems after azithromycin should not take it again.
Worsening myasthenia gravis
Azithromycin can worsen muscle weakness in people with myasthenia gravis and has been linked to new-onset myasthenic syndrome. Watch for drooping eyelids, double vision, trouble swallowing or breathing weakness.
Infants under six weeks: pyloric stenosis
Infantile hypertrophic pyloric stenosis, a thickening of the stomach outlet that causes forceful vomiting, has been reported after azithromycin in newborns treated up to 42 days of life. Parents should call promptly if a baby vomits repeatedly or is irritable with feeding. Even so, the CDC still considers azithromycin the drug of choice for whooping cough in very young infants, because the danger from pertussis outweighs this risk.
Less common effects listed by the label
- Headache, dizziness, vertigo and sleepiness
- Vaginal yeast infection (listed as monilia or vaginitis)
- Sensitivity to sunlight (photosensitivity), listed at 1% or less
- Palpitations and chest pain
- Rarely: kidney inflammation, low platelets, pancreatitis, seizures, anxiety or aggression, and tongue discoloration
Is It the Drug or the Infection?
Many “side effects” turn out to be symptoms of the illness itself. Telling them apart helps you avoid stopping a needed antibiotic, and it helps you spot when something new really is the drug.
| Symptom | Points toward the infection | Points toward azithromycin |
|---|---|---|
| Fatigue | Present before the first dose, improving as fever falls | New or worsening only after starting |
| Headache | Comes with fever and congestion | Starts soon after doses, without other illness signs |
| Nausea | Linked to coughing fits or postnasal drip | Peaks an hour or two after each dose |
| Rash | Viral rash often with fever early in illness | Hives, swelling or spreading rash after starting |
| Dizziness | From dehydration or fever | With palpitations or fainting, which needs urgent care |
When in doubt, call the pharmacist. They can often tell from timing alone which explanation is more likely.
Children: How Side Effects Differ
Kids get the same types of effects as adults, at rates that depend on the regimen. The label reports these figures for liquid azithromycin.
| Pediatric use and regimen | Diarrhea | Vomiting | Abdominal pain | Rash |
|---|---|---|---|---|
| Ear infection, 1-day | 4.3% | 4.9% | 1.4% | 1.0% |
| Ear infection, 3-day | 2.6% | 2.3% | 1.7% | 0.6% |
| Ear infection, 5-day | 1.8% | 1.1% | 1.2% | 0.4% |
| Pneumonia, 5-day | 5.8% | 1.9% | 1.9% | 1.6% |
| Strep throat, 5-day (12 mg/kg) | 5.4% | 5.6% | 3.4% | 0.7% |
The higher strep throat dose produces more vomiting than the ear-infection schedules. If your child vomits within minutes of a dose, call the pharmacy before giving more; the label says re-dosing after vomiting has not been formally established for the single large dose.
Symptom Decoder: What It Might Mean and What to Do
| What you notice | Likely explanation | Suggested action |
|---|---|---|
| Loose stools on days 1 to 4 | Gut motility and flora changes | Fluids, bland food; call if severe |
| Watery diarrhea with fever, weeks later | Possible C. difficile | Call your clinician promptly |
| Mild nausea after doses | Common GI effect | Take with food |
| Ringing ears, muffled hearing | Possible ototoxicity | Call prescriber same day |
| Racing or irregular heartbeat, fainting | Possible rhythm problem | Emergency care |
| Hives, face or throat swelling | Allergic reaction | Stop drug, emergency care |
| Yellow eyes, dark urine | Liver irritation | Stop drug, call prescriber |
| Vaginal itching or discharge | Yeast overgrowth | Ask pharmacist or clinician |
| Baby vomiting forcefully after feeds | Possible pyloric stenosis | Call the pediatrician urgently |
A Day-by-Day Picture of a Typical Short Course
Here is what many adults notice on a standard 5-day pack. It is not a promise; it is a pattern that helps you separate expected effects from warning signs.
- Day 1 (500 mg): The largest single dose of the course. If nausea or cramping is going to show up, it often appears a few hours after this dose. Taking it with food helps many people.
- Days 2 and 3 (250 mg each): Stomach effects, if any, usually stay mild. Loose stools may start as gut bacteria shift.
- Days 4 and 5: Most people feel the infection easing. Mild side effects tend to plateau rather than worsen.
- Days 6 to 10: The drug is still present at falling levels. Stomach effects usually fade. Any new rash, yellowing, hearing change or severe diarrhea in this window still counts as possibly drug-related.
- Weeks 2 to 8: Watch for late C. difficile diarrhea, which the label warns can appear more than two months after antibiotics.
Long-Term Azithromycin: A Different Side Effect Profile
Some people take azithromycin for months, for example to reduce COPD flare-ups, in bronchiectasis or cystic fibrosis care, or to prevent certain infections in people with weakened immunity. Long courses shift the risk balance:
- Hearing: cumulative exposure raises the chance of measurable hearing loss, which is why baseline and follow-up hearing checks are common.
- Heart rhythm: clinicians often check an ECG before starting long-term use, especially in older adults.
- Resistance: the 2011 COPD trial found that daily azithromycin increased colonization with macrolide-resistant bacteria.
- Liver tests: periodic blood work may be recommended.
Long-term use is a specialist decision that weighs fewer flare-ups against these risks. It is very different from a single short pack for an acute infection.
Lab Test Changes You Might Not Feel
In clinical trials, some adults had blood test changes during treatment, regardless of whether the drug caused them. The label lists, at rates above 1%, changes such as lower hemoglobin and white cell subtypes and higher liver enzymes (ALT, AST, GGT), creatine kinase, potassium, creatinine and blood sugar. When follow-up was done, the changes appeared reversible. Across more than 5,000 adults in multiple-dose trials, only four stopped because of liver enzyme abnormalities and one because of a kidney test change. These numbers explain why routine blood tests are not needed for a short course in most healthy people.
Pregnancy and Breastfeeding
The current label says that published studies and decades of postmarketing experience have not shown a drug-associated risk of major birth defects, miscarriage or adverse maternal or fetal outcomes. Azithromycin does pass into breast milk, and non-serious effects have been reported in breastfed infants. The label advises watching the baby for diarrhea, vomiting or rash. Side effects in the person taking the drug are the same as in anyone else.
Who Is More Likely to Have Problems
- Older adults: The label notes they may be more susceptible to torsades de pointes.
- People with heart rhythm problems or heart failure, or on drugs that prolong QT.
- People with low potassium or magnesium, which can follow vomiting, diarrhea or diuretic use.
- Anyone taking warfarin: postmarketing reports suggest azithromycin may increase its effect, so clotting times should be checked.
- People on long-term azithromycin for COPD, bronchiectasis or other conditions, who face cumulative hearing and resistance concerns.
- People with prior liver reactions to azithromycin, for whom it is contraindicated.
The label also advises caution with digoxin, colchicine and phenytoin, because other macrolides interact with them even though azithromycin trials did not report problems. Our comparison of azithromycin vs clarithromycin explains why clarithromycin has far more of these interactions.
Azithromycin vs Other Common Antibiotics: Side Effect Snapshot
| Antibiotic | Signature side effects |
|---|---|
| Azithromycin | GI upset, rare QT prolongation, rare hearing changes |
| Amoxicillin | Diarrhea, rash (including non-allergic rashes), allergy in a minority |
| Doxycycline | Sun sensitivity, esophageal irritation, nausea |
| Clarithromycin | Metallic taste, GI upset, many drug interactions |
For deeper comparisons, see amoxicillin side effects, amoxicillin vs azithromycin and azithromycin vs doxycycline.
How Long Do Side Effects Last?
Because azithromycin has a half-life of roughly 68 hours, it stays in your body for several days after the last tablet. Mild stomach effects usually fade within a few days of finishing. Allergic reactions can, unusually, persist or recur for longer. Hearing changes from long courses may or may not recover, which is why early reporting matters. C. difficile can begin after the course is over.
If you are curious how lingering drug levels compare with a short-acting antibiotic, our guide on how long amoxicillin stays in your system offers a helpful contrast.
Reducing Your Risk Before You Start
- Tell the prescriber about every medicine and supplement, especially heart, mood, nausea and HIV medicines, warfarin, digoxin and colchicine.
- Mention any personal or family history of sudden cardiac death, fainting or long QT.
- Report past reactions to erythromycin, clarithromycin or azithromycin.
- Mention liver disease, kidney failure, myasthenia gravis or hearing loss.
- Ask whether an antibiotic is really needed. Many coughs and sore throats are viral, and an antibiotic adds risk without benefit, a point we make in does azithromycin help bronchitis.
Reporting a Side Effect
If you experience a serious side effect, you or your doctor can report it to the FDA MedWatch program. The patient information on MedlinePlus lists both the online form and the phone number (1-800-332-1088). Reporting helps regulators spot patterns that individual doctors might miss.
Frequently Asked Questions
What is the most common side effect of azithromycin?
Diarrhea or loose stools, reported in about 4 to 5% of adults on standard multiple-dose courses, followed by nausea and abdominal pain.
Can azithromycin cause permanent hearing loss?
Hearing loss and tinnitus are listed in postmarketing reports. Most data come from long-term daily use, where a large trial found hearing decrements in 25% of treated patients versus 20% on placebo. With short courses it is rare. Report any change promptly.
Is azithromycin bad for your heart?
For most healthy people, the risk is very small. The FDA warns of a risk of QT prolongation and fatal arrhythmia, mainly in people with existing rhythm problems, low potassium or magnesium, slow heart rate or other QT-prolonging drugs.
Why do I still have diarrhea after finishing?
The drug lingers for days, and gut bacteria take time to rebalance. Mild loose stools usually settle. Watery diarrhea with fever or blood, especially starting after the course, needs medical evaluation for C. difficile.
Can azithromycin cause a yeast infection?
Yes. Vaginal yeast infection is listed among less common side effects because antibiotics can disturb normal vaginal bacteria.
Does azithromycin make you tired?
Fatigue and sleepiness appear in the label at 1% or less. Tiredness is more often from the infection itself.
Can azithromycin affect birth control?
Azithromycin is not known to reduce hormonal contraceptive effectiveness the way the antibiotic rifampin does, but vomiting or severe diarrhea can reduce absorption of the pill. Our article on antibiotics and birth control explains backup rules.
Can azithromycin cause dizziness or headache?
Yes, but uncommonly. Dizziness, vertigo and headache are listed at 1% or less in adults. Dizziness together with palpitations or fainting should be treated as a possible heart rhythm problem and checked urgently.
When should I go to the emergency room?
Go immediately for fainting, a new irregular or racing heartbeat, chest pain, swelling of the face or throat, difficulty breathing, blistering skin, or severe bloody diarrhea.
Sources
- DailyMed: Zithromax prescribing information (July 2026)
- FDA Drug Safety Communication on azithromycin and heart rhythm (2013)
- Albert et al., NEJM 2011: Azithromycin for prevention of COPD exacerbations
- Ray et al., NEJM 2012: Azithromycin and the risk of cardiovascular death
- CDC: Treatment of pertussis
- MedlinePlus: Azithromycin
Medical information on this page is educational. It is not a diagnosis or a substitute for care from a licensed clinician. If you think you are having a serious reaction, stop the medicine and seek urgent help.
