Amoxicillin

Is amoxicillin safe?

Yes, for nearly everyone who is not truly allergic to penicillin. Amoxicillin has been used since the 1970s in hundreds of millions of people, from newborns to the very old and in pregnancy. It has no boxed warningBoxed warningThe strongest safety warning the US FDA puts on a medicine's label, printed inside a black border at the top.More in the glossary, no dependence risk and a very wide overdose marginOverdose marginHow far a normal dose is from a harmful one. A wide margin means an accidental extra dose is unlikely to cause serious harm.More in the glossary. We score it 91, just below loratadine (92) and above doxycycline (86).

The everyday side effects are an upset stomach, diarrhoea and rash. The rare serious one is a severe allergic reaction, and that is the reason to tell every prescriber about any past reaction to a penicillin.

How common is a real allergy?

Much less common than people think. The CDC says about 10% of US patients report a penicillin allergy, but fewer than 1% are truly allergic when evaluated. A 2019 JAMA review adds that about 80% of people with a genuine immediate (IgE) allergy lose it within a decade. Many old "allergies" were viral rashes, a family history, or side effects such as an upset stomach.

The label matters because people tagged as allergic get broader, more expensive antibiotics that cause more C. difficile infections and more resistant bacteria such as MRSA. If you carry a penicillin allergy label from childhood, ask your doctor whether testing makes sense. For people with a low-risk history, the JAMA review supports a supervised test dose of amoxicillin; a negative skin test followed by a test dose is close to 100% reliable at ruling allergy out.

True anaphylaxis is rare: reviews estimate 1 to 5 cases per 10,000 courses of penicillin, and it is less common with tablets than injections. It usually starts within an hour of a dose with hives, swelling of the lips or throat, wheezing or faintness. Call emergency services if this happens. The label also lists rare severe skin reactions, such as Stevens-Johnson syndrome and DRESS; a rash that blisters, peels or comes with fever needs urgent care.

Cross-reaction with cephalosporin antibiotics is about 2%, much lower than the 8% once quoted. Since January 2025 UK guidance classes first-generation cephalosporins as first-choice "Access" antibiotics, widening the options for people who are truly allergic.

Side effects that are common but rarely serious

• Diarrhoea, nausea and vomiting. The label lists these among the most common reactions. Taking it at the start of a meal helps.
• Rash. Often a flat, non-itchy rash that is not allergic. The label says a high percentage of people with glandular fever (mononucleosis) get one, which is why it should not be used in that illness.
• Thrush. Vaginal or oral yeast infections are common after antibiotics.

C. difficile: the delayed risk

Any antibiotic thins out the normal gut bacteria and can let C. difficile take hold. The CDC says people are up to 10 times more likely to get it while taking an antibiotic and in the month afterwards, and longer courses can double the risk. The amoxicillin label warns it can range from mild diarrhoea to fatal colitis and has been reported more than 2 months after a course. Watery diarrhoea that is severe, bloody or lasting more than a few days after antibiotics needs a doctor's check; the NHS advises not to take other medicines for diarrhoea without speaking to a pharmacist or doctor.

Does amoxicillin add to antibiotic resistance?

Every course does a little, which is why the label says not to use it without a proven or strongly suspected bacterial infection. Plain amoxicillin sits in the WHO's "Access" group, the narrower first-choice antibiotics that are least likely to drive resistance; co-amoxiclav is classed "Watch" in the UK. England aims for 70% of antibiotic use to come from the Access group, but managed 63.2% in 2024, according to UKHSA. Resistant bloodstream infections rose 22.7% between 2019-20 and 2024-25 there, with E. coli the most common cause.

Doses

Patient	Dose on the US label
Adults, mild to moderate infection	500 mg every 12 hours or 250 mg every 8 hours
Adults, severe or chest infection	875 mg every 12 hours or 500 mg every 8 hours
Children over 3 months, under 40 kg	20 to 45 mg per kg per day in divided doses
H. pylori (with other drugs)	1 g two or three times a day for 14 days

In severe kidney disease (GFR under 30 mL/min) the label rules out the 875 mg tablet and uses 250 or 500 mg every 12 hours, or every 24 hours below 10 mL/min. Overdose is rarely dangerous: in a study of 51 children, amounts under 250 mg per kg did not cause significant symptoms, though large overdoses can leave crystals in the urine and briefly injure the kidneys. Still call Poison Control (1-800-222-1222) or NHS 111 if a child takes extra.

Amoxicillin versus Augmentin

Augmentin, called co-amoxiclav in the UK, adds clavulanic acid to amoxicillin so it works against more bacteria. It causes more diarrhoea, and it is a known cause of drug-induced liver injuryDrug-induced liver injuryLiver damage caused by a medicine, herbal product or supplement. It is usually rare and unpredictable, but it can be serious.More in the glossary, which is why its label contraindicates it in anyone who has had jaundice from it before. This page scores plain amoxicillin only.

Who should be careful

• Anyone with a past reaction to a penicillin or cephalosporin: tell the prescriber exactly what happened.
• People on warfarin: amoxicillin can raise the INR, so an extra blood test may be needed.
• People taking methotrexate or allopurinol, or with severe kidney disease.
• Pregnant or breastfeeding women: animal studies showed no harm and it is widely used, but it passes into breast milk, so mention it to the baby's doctor if a rash or diarrhoea appears.

Antibiotics do nothing for colds, flu or most sore throats, which are caused by viruses. It is reasonable to ask your doctor or pharmacist whether you really need one.

How we scored it

Overdose marginOverdose marginHow far a normal dose is from a harmful one. A wide margin means an accidental extra dose is unlikely to cause serious harm.More in the glossary (95), dependence (98), track record (96) and the lack of a boxed warningBoxed warningThe strongest safety warning the US FDA puts on a medicine's label, printed inside a black border at the top.More in the glossary (92) are all strong. Serious side effects (85) is the lowest factor because of rare anaphylaxis, severe skin reactions and C. difficile. For a stomach medicine often given alongside amoxicillin for H. pylori, see omeprazole.