Antibiotic Side Effects in Kids and Adults: What’s Normal, What’s Not, and When to Call Your Pharmacist

Last Reviewed: August 2, 2026

Antibiotics are among the most commonly prescribed medicines for both children and adults, and among the most misunderstood when it comes to side effects. A little stomach upset has a parent googling at midnight. A rash three days into a course leaves an adult wondering whether to stop the medicine or push through. And sometimes the real warning signs, the ones that need a same-day call, slip by because they don’t feel obviously “severe.”

As a pharmacist and mom, I get some version of this question all the time: is this normal, or do I need to call someone? This guide walks through exactly that, side effect by side effect and class by class, for kids and adults.

If your child has just started their first antibiotic for an ear infection, start with Antibiotics for Ear Infections in Kids: What Parents Need to Know, which covers when antibiotics are actually needed. This post picks up from there and goes deeper into what happens once you start the medicine, for every antibiotic class, at every age.

Side Effect, Allergy, or Something Else? Know the Difference First

Before getting into specific drugs, it helps to understand that “antibiotic side effects” actually covers three different things, and they’re not treated the same way.

A side effect is an expected, non-allergic reaction that happens because of how the drug works in your body, most often gut-related. Antibiotics don’t just target the bacteria making you sick; they also disrupt the healthy bacteria living in your gut and, in women, the vagina. That disruption is the root cause of most “normal” antibiotic side effects: loose stools, mild diarrhea, nausea, and yeast infections.

A true allergy is an immune-system response. Your body treats the antibiotic as a threat and reacts, sometimes mildly (hives, itching) and sometimes severely (anaphylaxis). Allergic reactions usually show up within minutes to hours of a dose, though delayed reactions over several days are also possible.

A drug interaction or class-specific risk is a separate category entirely: things like tendon damage from fluoroquinolones or heart-rhythm changes from certain macrolides that aren’t allergies and aren’t typical GI side effects. These are tied to how a specific class of antibiotic behaves in the body.

Knowing which bucket a symptom falls into changes what you do next, which is exactly why the rest of this guide is organized by antibiotic class.

What’s Normal: Expected Side Effects Across Most Antibiotics

Regardless of which antibiotic is prescribed, a few side effects are common enough across the board that they’re considered a normal part of treatment for most people, kids and adults alike:

• Mild nausea or stomach upset, especially if the dose is taken on an empty stomach
• Loose stools or mild diarrhea, from disruption of healthy gut bacteria. In babies, these loose stools can trigger diaper rash too.
• Mild loss of appetite, particularly in younger kids
• A metallic or unusual taste in the mouth (common with metronidazole and clarithromycin)
• Vaginal yeast infections or oral thrush, because antibiotics kill protective bacteria along with the harmful ones
• Mild, non-spreading rash without other symptoms, though this one always deserves a call to confirm it isn’t the start of something more

These symptoms are uncomfortable but generally don’t mean the antibiotic needs to be stopped. If your child is having loose stools while on antibiotics, my guide on how to treat vomiting and diarrhea in kids walks through safe home management and hydration, and Kids and Dehydration: Quick Guide covers the signs that mean you need to seek care instead.

Antibiotic Side Effects by Class

Antibiotics aren’t interchangeable, and neither are their side effect profiles. Here’s what to expect from the classes most commonly prescribed to kids and adults.

Penicillins (Amoxicillin, Amoxicillin-Clavulanate / Clavulin, Penicillin V)

The most frequently prescribed class for kids, and generally one of the best tolerated.

• Common: diarrhea, nausea, mild stomach cramps. Amoxicillin-clavulanate (Clavulin) causes noticeably more diarrhea than amoxicillin alone because of the clavulanate component.
• Less common but important: skin rash. Not every rash on amoxicillin is a true allergy; a common, harmless, non-itchy pink rash can appear if a child has a viral infection at the same time (research documents this well, and it doesn’t necessarily mean avoiding penicillins in the future). Still, report any new rash to your pharmacist or doctor, who can assess it properly rather than leaving you to guess.
• Why proper assessment matters so much: the Canadian Medical Association Journal notes that about 10 percent of people report a penicillin allergy, but 90 to 95 percent of them are not truly allergic when formally evaluated. Much of this comes from intolerances (like nausea) being recorded as allergies, or from allergy fading over time. An incorrect penicillin allergy label follows a child into adulthood and pushes them toward broader, sometimes less effective antibiotics for life. If a rash appears, take photos, note the timing relative to the first dose, and have it assessed rather than simply recording “penicillin allergy” forever.
• Watch for: hives, facial or lip swelling, throat tightness, or trouble breathing, true signs of an allergic reaction that need urgent care.

Cephalosporins (Cephalexin, Cefuroxime, Cefprozil, Cefixime)

Often used when penicillin isn’t tolerated or appropriate for the infection.

• Common: diarrhea, nausea, stomach upset, similar to penicillins.
• Less common: rash, and in rare cases, changes in blood counts or clotting with certain cephalosporins.
• Cross-reactivity note: people with a penicillin allergy have only a small chance (commonly cited around 1 to 2 percent) of also reacting to a cephalosporin, but your prescriber and pharmacist should always know about any prior antibiotic allergy before a cephalosporin is started.

Macrolides (Azithromycin, Clarithromycin, Erythromycin)

Frequently used for respiratory infections and in people with penicillin allergies.

• Common: nausea, diarrhea, stomach cramping, and a bitter or metallic taste. Erythromycin is the most stomach-unfriendly of the three.
• Important for adults, especially those with heart conditions: macrolides, particularly azithromycin, carry both an FDA warning and a matching Health Canada safety advisory for QT prolongation, a heart-rhythm change that can be serious in people with existing heart rhythm problems, low potassium or magnesium, or those taking other QT-prolonging medications. Symptoms to watch for include a racing or irregular heartbeat, dizziness, or fainting.
• Drug interactions: macrolides interact with many other medications, including some statins. Always tell your pharmacist about everything else you’re taking, including supplements. My post on drug interactions hiding in plain sight covers more combinations like this.

Fluoroquinolones (Ciprofloxacin, Levofloxacin, Moxifloxacin)

Reserved for more serious or resistant infections, and generally not the first choice for children or for straightforward infections in adults.

• Common: diarrhea, nausea, dizziness, headache.
• Most serious warnings: the FDA lists a Boxed Warning, its strongest kind, and Health Canada issued a matching Summary Safety Review. The risks include tendinitis and tendon rupture, most often in the Achilles tendon at the back of the ankle. They also include peripheral neuropathy: nerve pain, tingling, numbness, or weakness that can become permanent. Rarely, these drugs affect the central nervous system, causing confusion or seizures. Health Canada’s review matched the FDA’s and updated safety information for all fluoroquinolones sold in Canada. Tendon problems can appear during the course or even weeks after finishing.
• In adults: the FDA has specifically advised avoiding fluoroquinolones for uncomplicated infections like sinusitis, bronchitis, and simple UTIs when other options exist, and Health Canada issued parallel guidance limiting their use for uncomplicated infections, because the risk of serious side effects can outweigh the benefit for milder infections.
• In kids: fluoroquinolones are used more cautiously and reserved for situations where the benefit is clear. This caution originally stemmed from concerns about effects on developing joints, but human studies in children have been more reassuring, with most finding no clear increase in joint or tendon problems.
• Absorption note: dairy, calcium, iron, magnesium, and antacids bind fluoroquinolones in the gut and reduce absorption. Separate them by at least two hours, and confirm the exact gap with your pharmacist.
• Call right away if you or your child develops sudden tendon pain, swelling, or difficulty moving a joint while on this medication, and stop activity on that limb until you’ve spoken with your provider.

Sulfonamides (Sulfamethoxazole-Trimethoprim / Septra)

Commonly used for UTIs and certain skin infections.

• Common: nausea, vomiting, loss of appetite, sun sensitivity.
• Serious skin reactions: among antibiotics, sulfonamides carry one of the highest risks for Stevens-Johnson syndrome and toxic epidermal necrolysis, rare but life-threatening skin reactions. Warning signs include a spreading rash, blistering or peeling skin, and sores in the mouth, eyes, or on the lips. This class should be stopped immediately and medical care sought at the first sign of a rash that seems to be progressing rather than improving.
• Not for young infants: sulfonamides are generally avoided in infants under 2 months and near the end of pregnancy due to a risk of severe newborn jaundice (kernicterus).
• Interaction note: this class interacts with several medications, including warfarin.

Tetracyclines (Doxycycline, Minocycline)

• Common: nausea, stomach upset, and photosensitivity, meaning your skin burns more easily in the sun. Sun protection matters more than usual on this medication; my sunscreen guide walks through choosing one properly.
• Practical tips: take doxycycline with a full glass of water, and stay upright for 30 minutes afterward. If it lingers, it can irritate the esophagus. Also separate it from dairy, calcium, iron, and antacids by at least two hours. These bind the drug and reduce how much you absorb.
• The tooth-staining question: older tetracyclines permanently stain developing teeth in children under 8. For decades, that risk kept this class out of young kids. Doxycycline binds calcium less readily, though. Current research shows short courses, under about three weeks, don’t cause visible staining in young children. The Canadian Paediatric Society and the American Academy of Pediatrics now back short-course doxycycline for children under 8 when indicated. Pediatricians prescribe it more comfortably than before, but it’s still a fair question to ask your prescriber.
• Not for use in pregnancy beyond early, brief, medically necessary use, due to effects on fetal bone and tooth development.

Lincosamides (Clindamycin / Dalacin)

• Common: diarrhea, nausea, metallic taste.
• Important: Clindamycin carries the highest risk of any commonly used antibiotic for Clostridioides difficile (C. diff) associated diarrhea and colitis, a serious gut infection that can occur during treatment or even weeks to months after finishing the course. The Public Health Agency of Canada identifies C. diff as the most common cause of antibiotic-associated diarrhea in Canadian healthcare settings. Watch for watery diarrhea (especially more than a few episodes a day), abdominal pain or cramping, fever, or blood in the stool, and call your provider promptly if these develop, even after the antibiotic is done. Do not take an anti-diarrheal like loperamide (Imodium) for suspected C. diff without medical advice, because slowing the gut can make it worse.

Nitrofurantoin (Macrobid)

An adult-focused antibiotic used almost exclusively for urinary tract infections, included here because it’s one of the most commonly prescribed antibiotics for women and has a side effect profile that’s easy to miss.

• Common: nausea (take it with food), headache, and urine that may turn dark yellow or brown, which is harmless and reverses when the course ends. I mention the urine change because it genuinely alarms people who were not warned.
• Avoid in late pregnancy (at term) and in people with significant kidney impairment, since the drug needs to concentrate in the urine to work and won’t be effective or safe if kidneys aren’t clearing it properly.

Metronidazole (Flagyl)

• Common: metallic taste, nausea, stomach upset, and dark urine (harmless).
• Alcohol warning: avoid alcohol during treatment and for a period after finishing (commonly advised as 24 to 72 hours), due to a disulfiram-like reaction that can cause flushing, rapid heartbeat, nausea, and vomiting. This includes hidden sources like some mouthwashes and cough syrups.

Adult-Specific Considerations

A few things come up specifically for adults that don’t apply the same way to kids.

Antibiotics and birth control. With the notable exception of rifampin and its relative rifabutin (used mainly for tuberculosis and a few other specific situations), most antibiotics, including amoxicillin, do not reduce the effectiveness of hormonal birth control. This is a persistent myth; a 2018 systematic review of non-rifamycin antibiotics found no meaningful reduction in contraceptive effectiveness, and both the Society of Obstetricians and Gynaecologists of Canada and the UK Faculty of Sexual and Reproductive Healthcare advise that you don’t need backup contraception with antibiotics that don’t induce liver enzymes. Two caveats: if your prescriber puts you on rifampin or rifabutin, ask your pharmacist about backup contraception, and if a course leaves you vomiting or with significant diarrhea, that alone can affect how well you absorb the pill, so backup protection makes sense until things settle.

Pregnancy and breastfeeding. Antibiotic safety varies significantly by class and trimester; prescribers generally count penicillins and cephalosporins among the safer options, and usually avoid tetracyclines and, in most cases, fluoroquinolones. Many common antibiotics are compatible with breastfeeding, but this is drug-specific; your pharmacist can check it against specialized pregnancy and lactation references before you start.

Drug interactions. Adults are more likely to be on other regular medications, and antibiotics are a common source of interactions. Sulfamethoxazole-trimethoprim and metronidazole, for example, can both increase the effect of the blood thinner warfarin, raising bleeding risk, which is why INR monitoring often increases during a course. Always bring your full medication and supplement list to your pharmacist when starting something new.

Older adults. Age-related changes in kidney and liver function affect how antibiotics are cleared from the body, which is part of why nitrofurantoin, in particular, is used more cautiously in older adults.

Kids-Specific Considerations

Dosing accuracy matters more than the drug itself. Many “side effects” reported in kids are actually dosing errors: too much medication measured with a kitchen spoon, or doses given too close together. If you haven’t already, my guide on common children’s medication mistakes covers the errors I see most often, including reusing leftover antibiotics from a previous illness, which is never appropriate even if the symptoms look similar.

Managing discomfort alongside the antibiotic. Fever, ear pain, or a sore throat often needs its own pain relief plan, separate from the antibiotic itself. The antibiotic clears the infection, but a fever or pain reliever is what eases the symptoms while it works. Your pharmacist can help you choose the right one and dose it correctly for your child’s weight.

Diarrhea and dehydration risk. Because GI side effects are so common in kids on antibiotics, dehydration is the thing I’d actually keep the closest eye on, not the diarrhea itself. Review the signs in Kids and Dehydration: Quick Guide so you know what warrants a call versus what you can manage with fluids at home.

Reading the label correctly reduces errors. If you’re ever unsure about dose, frequency, or how a liquid antibiotic should be measured, my guide on how to read a prescription label like a pharmacist walks through exactly what each section means and when to call your pharmacist for clarification.

What’s Not Normal: When to Call Your Pharmacist or Doctor Today

Call your pharmacist or prescriber the same day (not urgently, but promptly) if you or your child experience:

• A rash that’s spreading or that appears alongside itching all over the body
• Diarrhea that’s more frequent, watery, or lasts longer than expected, especially if it starts or continues after the antibiotic course ends
• Vomiting that prevents keeping the antibiotic down
• No improvement in the original infection after 48 to 72 hours on the medication, or a fever that returns after initially improving

When to Seek Emergency Care

Go to urgent care or the emergency room, or call 911, for:

• Signs of anaphylaxis: throat tightness, difficulty breathing or swallowing, swelling of the face, lips, or tongue, wheezing, or fainting
• Signs of a severe skin reaction: blistering or peeling skin, sores in the mouth or eyes, or a rash accompanied by fever and facial swelling (possible Stevens-Johnson syndrome, most associated with sulfonamides)
• Signs of severe C. diff colitis: severe abdominal pain, high fever, and frequent watery or bloody diarrhea (most associated with clindamycin, though possible with any antibiotic)
• Sudden tendon pain, swelling, or inability to bear weight on a joint (associated with fluoroquinolones)
• A racing, pounding, or irregular heartbeat, or fainting, particularly in someone on a macrolide or fluoroquinolone with existing heart risk factors
• In babies and young kids: refusing fluids, noticeably fewer wet diapers, or unusual sleepiness or floppiness

How to Reduce Side Effects While Still Finishing the Course

• Take antibiotics with food if the label allows it; this reduces nausea for most classes
• Stay well hydrated, especially if GI side effects appear
• Space probiotic supplements at least 2 hours apart from antibiotic doses if you choose to use them; a Cochrane review found probiotics may reduce the risk of antibiotic-associated diarrhea in children, and they’re generally low-risk for most healthy people, though anyone immunocompromised should check with their pharmacist first
• Avoid alcohol entirely with metronidazole and related drugs
• Use sun protection on tetracyclines and, to a lesser degree, fluoroquinolones and sulfonamides
• Separate dairy, calcium, iron, and antacids from tetracyclines and fluoroquinolones by at least two hours
• Finish the full course unless your prescriber tells you otherwise. Stopping early because side effects felt “normal enough to push through” is different from stopping early because of a genuine warning sign. If you’re unsure which one you’re dealing with, that’s exactly what your pharmacist is there for.

Clinical Mama Takeaway

Most antibiotic side effects, stomach upset, loose stools, a mild taste change, are the medicine doing what antibiotics do: upsetting your bacteria, including some of the helpful kind. That’s uncomfortable, but it’s expected, and it’s rarely a reason to stop the medicine on your own.

What deserves a same-day call or urgent care is different for each class: tendon symptoms on a fluoroquinolone, a spreading blistering rash on a sulfa drug, watery diarrhea weeks after finishing clindamycin, an irregular heartbeat on a macrolide. Knowing which class you or your child is on, and what that class is watched for, is the difference between a confident phone call and a needless trip to the ER, or worse, a missed warning sign.

When in doubt, your pharmacist is the fastest, easiest person to reach. That’s what we’re here for.

❓ Frequently Asked Questions

📚 References

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