Trimethoprim is an antibiotic.

It's used to treat urinary tract infections (UTIs), such as cystitis.

Occasionally, trimethoprim is used to treat other types of infections, such as chest infections and acne.

Trimethoprim is available on prescription. It comes as tablets and as a liquid that you drink.

• Trimethoprim is usually taken twice a day to treat infections.
• For most infections, you'll feel better within a few days.
• Side effects may include itching or a mild skin rash, but these are usually mild and short-lived.
• You can drink alcohol while taking trimethoprim.
• There are no brand names for this medicine at the moment.

Trimethoprim can be taken by adults and children.

Trimethoprim isn't suitable for some people. To make sure this medicine is safe for you, tell your doctor if you:

• have ever had an allergic reaction to trimethoprim or any other medicines in the past
• have liver or kidney problems
• have anaemia or low amounts of folic acid (folate) in your blood
• have porphyria (a rare inherited blood disorder) or any other blood disorder
• are trying to get pregnant or already pregnant

Trimethoprim is usually taken twice a day to treat an infection - once in the morning and once in the evening. You can take it with or without food.

The usual dose of trimethoprim to:

• treat UTIs is 200mg twice a day - your doctor might recommend you double the first dose to 400mg
• prevent infections is 100mg once a day
• treat cystitis that comes on after having sex is a one-off dose of 100mg
• treat acne is 300mg twice a day - this dose might be reduced over time

The dose of trimethoprim you need to take depends on your illness, your age, and how well your kidneys work.

Doses are usually lower for elderly people and those with kidney problems.

How to take it

Swallow trimethoprim tablets whole with a drink of water. Do not chew or break them.

Trimethoprim is available as a liquid for people who find it difficult to swallow tablets.

If you're taking trimethoprim as a liquid, it'll usually be made up for you by your pharmacist. The medicine will come with a syringe or spoon to help you take the right amount.

If you don't have a syringe or spoon, ask your pharmacist for one. Do not use a kitchen teaspoon as it will not give the right amount.

If you're taking trimethoprim to prevent an infection, take it at bedtime.

If you have been prescribed trimethoprim as a treatment for cystitis that comes on after having sex, take it as a single dose within 2 hours of having sex (no more than twice a day).

How long to take it for

The length of time you'll need to take trimethoprim for depends on how bad and where your infection is, your age, whether you're male or female, and whether you have any other health problems.

• Women with straightforward UTIs usually take a 3-day course of treatment.
• Men and pregnant women with straightforward UTIs usually take a 14-day course of treatment.
• People with particularly severe or complicated UTIs, or a catheter, usually take a 14-day course of treatment.
• A treatment course for 4 to 6 weeks could be needed if the UTI causes swelling of the prostate gland in men (prostatitis).
• Treatment may continue for at least 6 months for preventing UTIs or as a treatment for acne.

It's very important that you continue taking trimethoprim until your course is finished, even if you feel better, to help stop the infection coming back.

What if I forget to take it?

If you forget to take a dose, take it as soon as you remember, unless it's nearly time for your next dose. In this case, just leave out the missed dose and take your next dose as normal.

Never take 2 doses at the same time. Never take an extra dose to make up for a forgotten one.

If you forget doses often, it may help to set an alarm to remind you. You could also ask your pharmacist for advice on other ways to help you remember to take your medicine.

What if I take too much?

Taking an extra dose of trimethoprim by accident is unlikely to harm you, but it may increase the chances of temporary side effects, such as feeling or being sick and diarrhoea.

Talk to your doctor or pharmacist if you:

• are worried or get severe side effects
• have taken more than 1 extra dose

You're unlikely to get side effects from trimethoprim. Some people get itching or a skin rash, but this is usually mild and goes away after you stop taking the medicine.

Common side effects

The most common side effects with trimethoprim are itching or a mild rash. They happen in more than 1 in 100 people.

Other side effects of trimethoprim are:

• feeling sick
• diarrhoea
• headaches

Talk to your doctor or pharmacist if the side effects bother you or don't go away.

Serious side effects

Serious side effects are rare and happen in less than 1 in 1,000 people.

Call a doctor straight away if you have:

• muscle weakness, an abnormal heartbeat, chest pains, or are feeling or being sick (vomiting) - these can be signs of high potassium in your blood
• serious skin reactions or rashes, including irregular, round red patches, peeling, blisters, skin ulcers, or swelling of the skin that looks like burns
• headaches, fever, stiff neck, tiredness, feel ill, and your eyes become very sensitive to bright light - these can be signs of meningitis
• diarrhoea (possibly with stomach cramps) that contains blood or mucus - if you have severe diarrhoea that lasts longer than 4 days, you should also speak to a doctor
• bruising or bleeding you can't explain (including nosebleeds), a sore throat, mouth ulcers, a high temperature, or you feel tired or generally unwell - these can be signs of a problem with your blood

Call 999 or go to A&E straight away if you have headaches, fever, a stiff neck, tiredness, feel ill, and your eyes become very sensitive to bright light - these can be signs of meningitis.

Serious allergic reaction

In rare cases, it's possible to have a serious allergic reaction to trimethoprim.

These aren't all the side effects of trimethoprim.

For a full list, see the leaflet inside your medicine packet.

What to do about:

• itching or a mild rash - it may help to take an antihistamine, which you can buy from a pharmacy. Check with the pharmacist to see what type is suitable for you.
• feeling sick- try taking trimethoprim with or after food to see if that helps ease your symptoms. It may also help if you avoid rich or spicy food while you're taking this medicine.
• diarrhoea - drink plenty of water to avoid dehydration. Signs of dehydration include peeing less than usual or having dark, strong-smelling pee. Do not take any other medicines to treat diarrhoea without speaking to a pharmacist or doctor.
• headaches - make sure you rest and drink plenty of fluids. Do not drink too much alcohol. Everyday painkillers, such as paracetamol and ibuprofen, are safe to take with trimethoprim.

Trimethoprim isn't the safest antibiotic to take in pregnancy. Doctors generally agree you should take it only if the benefits outweigh the risks.

It's been linked with a small risk of problems for the unborn baby if it's taken in early pregnancy.

A substance called folic acid is important for the normal development of an unborn baby. Pregnant women are routinely advised to take a 400mcg folic acid supplement every day for the first 12 weeks.

Trimethoprim lowers levels of folic acid in the bloodstream. If you take this medicine during the first 12 weeks of pregnancy, your doctor will probably prescribe a high dose of folic acid (5mg daily) for you to take along with the trimethoprim.

There are no known risks to a pregnant woman or her unborn baby from taking trimethoprim after the first 12 weeks.

For more information about how trimethoprim can affect you and your baby during pregnancy, read this leaflet on the Best Use of Medicines (BUMPS) website.

Trimethoprim and breastfeeding

You can breastfeed while taking trimethoprim. Trimethoprim passes into breast milk, but only in tiny amounts that aren't harmful to the baby.

Talk to your pharmacist or doctor if you're worried.

There are many medicines that don't mix well with trimethoprim.

Tell your doctor if you're taking these medicines before starting trimethoprim:

• an antibiotic called rifampicin
• a blood thinner, such as warfarin
• digoxin (a heart medicine)
• phenytoin (an epilepsy medicine)
• diabetes medicines called replaglinide and pioglitazone

Typhoid vaccine given by mouth may not work properly if you're taking trimethoprim. This doesn't apply to typhoid vaccines given by injection.

Mixing trimethoprim with herbal remedies and supplements

There are no known problems with taking herbal remedies and supplements with trimethoprim.