Conditions

Antibiotics can sometimes interact with other medicines or substances. This means it can have an effect that is different to what you expected.

If you want to check that your medicines are safe to take with your antibiotics, ask your GP or local pharmacist.

Some antibiotics need to be taken with food, while others need to be taken on an empty stomach. Always read the patient information leaflet that comes with your medicine.

Alcohol

It's best to completely avoid alcohol while taking metronidazole or tinidazole, and for 48 hours afterwards, as this combination can cause very unpleasant side effects, such as:

• feeling and being sick
• stomach pain
• hot flushes
• headaches

It's recommended that you do not drink alcohol while taking antibiotics in general. However, as long as you drink in moderation, alcohol is unlikely to interact significantly with your medicine.

Read more about drinking alcohol while taking antibiotics.

The contraceptive pill

Some antibiotics, such as rifampicin and rifabutin, can reduce the effectiveness of the contraceptive pill.

If you're prescribed rifampicin or rifabutin, you may need to use additional contraception, such as condoms, while taking antibiotics. Speak to your GP, nurse or pharmacist for advice.

Mixing medicines

Some of the medicines you may need to avoid, or seek advice on, while taking an antibiotic include:

Penicillins

It's usually recommended that you avoid taking penicillin at the same time as methotrexate, which is used to treat psoriasis, rheumatoid arthritis and some forms of cancer. This is because combining the 2 medications can cause a range of unpleasant and sometimes serious side effects.

However, some forms of penicillin, such as amoxicillin, can be used in combination with methotrexate.

You may experience a skin rash if you take penicillin and allopurinol, which is used to treat gout.

Cephalosporins

Cephalosporins may increase the chance of bleeding if you're taking blood-thinning medications (anticoagulants) such as heparin and warfarin.

If you need treatment with cephalosporins, you may need to have your dose of anticoagulants changed or additional blood monitoring.

Aminoglycosides

The risk of damage to your kidneys and hearing is increased if you're taking 1 or more of the following medications:

• antifungals – used to treat fungal infections
• cyclosporin – used to treat autoimmune conditions such as Crohn's disease and given to people who have had an organ transplant 
• diuretics – used to remove water from the body
• muscle relaxants

The risk of kidney and hearing damage has to be balanced against the benefits of using aminoglycosides to treat life-threatening conditions such as septicaemia.

In hospital, blood levels are carefully monitored to ensure there's a safe amount of the antibiotic in the blood.

These side effects do not happen with aminoglycoside creams and eardrops if they're used properly.

Tetracyclines

Check with your GP or pharmacist before taking a tetracycline if you're currently taking:

• vitamin A supplements
• retinoids – such as acitretin, isotretinoin and tretinoin, which are used to treat severe acne
• blood-thinning medication
• diuretics
• kaolin-pectin and bismuth subsalicylate – used to treat diarrhoea
• medicines to treat diabetes – such as insulin
• atovaquone – used to treat pneumonia
• antacids – used to treat indigestion and heartburn
• sucralfate – used to treat ulcers
• lithium – used to treat bipolar disorder and severe depression
• digoxin – used to treat heart rhythm disorders
• methotrexate
• strontium ranelate – used to treat osteoporosis
• colestipol or colestyramine – used to treat high cholesterol
• ergotamine and methysergide – used to treat migraines

Macrolides

Do not take a macrolide antibiotic with any of the following medications unless directly instructed to by your GP, as the combination could cause heart problems:

• terfenadine, astemizole and mizolastine – these are all antihistamines used to treat allergic conditions such as hay fever
• amisulpride – used to treat episodes of psychosis
• tolterodine – used to treat urinary incontinence
• statins – used to treat high cholesterol

Fluoroquinolones

Check with your GP or pharmacist before taking a fluoroquinolone if you're currently taking:

• theophylline – used to treat asthma; also found in some cough and cold medicines
• non-steroidal anti-inflammatory drug (NSAID) painkillers – such as ibuprofen
• ciclosporin
• probenecid – used to treat gout
• clozapine – used to treat schizophrenia
• ropinirole – used to treat Parkinson's disease
• tizanadine – used to treat muscle spasms
• glibenclamide – used to treat diabetes
• cisapride – used to treat indigestion, heartburn, vomiting or nausea
• tricyclic antidepressants – such as amitriptyline
• steroids (corticosteroids) – such as prednisolone)

Some fluoroquinolones can intensify the effects of caffeine (a stimulant found in coffee, tea and cola), which can make you feel irritable, restless and cause problems falling asleep (insomnia).

You may need to avoid taking medication that contains high levels of minerals or iron, as this can block the beneficial effects of fluoroquinolones. This includes:

• antacids
• zinc supplements
• some types of multivitamin supplements