Trimethoprim
Trimethoprim is an antibiotic used to treat and prevent urinary tract infections such as cystitis, and occasionally other infections. It interferes with folic acid production in bacteria, and the NHS says it is not usually recommended in early pregnancy because it can lower folic acid levels. The NHS lists blood and skin reactions as rare serious side effects that need prompt medical help.
- Generic name
- Trimethoprim
- Active ingredient
- Trimethoprim
- Product type
- Small-molecule medicine
- Drug class
- Urinary antibacterials
- Brand names
- UK Trimethoprim · US
- Routes
- Oral
- Dosage forms
- Tablet, Suspension
- Usual preparations
- Supplied as tablets and as an oral suspension. (summary of the cited reference, not a prescription)
- Monitoring
- A long course of trimethoprim can occasionally lower folic acid levels, according to the NHS, and a doctor may change the antibiotic or prescribe high-dose folic acid for someone who takes it daily while trying to conceive.
Availability by country
| Country | Status | Note | Source |
|---|---|---|---|
| UK | Prescription only | UK |
Legal status differs between countries and can change; each row cites the document it was taken from and applies only to the country named.
What it is used for
- Treating urinary tract infections, such as cystitis
- Preventing urinary tract infections
- Occasionally other infections, including chest infections and acne
How it works in the body
Trimethoprim works by interfering with the way bacteria make folic acid. Antibiotics such as this act on bacteria and do not work against colds, flu or other viral infections. In the body, a long course can occasionally lower folic acid levels.
Common side effects
- Itching or a mild rash, feeling or being sick, diarrhoea, thrush and headaches are the common effects listed by the NHS.
- Being sick or having severe diarrhoea can stop contraceptive pills from protecting against pregnancy.
- For headaches, the NHS lists paracetamol and ibuprofen as suitable to take alongside trimethoprim.
Serious: seek help
- Unexplained bruising or bleeding, a sore throat, mouth ulcers or a high temperature can be signs of a blood problem; the NHS says to contact NHS 111 or a doctor straight away.
- Serious skin reactions such as blistering or peeling need the same prompt contact with NHS 111 or a doctor.
- Bloody or severe diarrhoea, sudden severe stomach pain (a possible sign of pancreatitis) and yellowing of the skin or eyes (a possible sign of liver problems) also need prompt contact with NHS 111 or a doctor.
- Signs of meningitis (such as headache, high temperature, stiff neck and sensitivity to bright light), which the NHS describes as very rare, or of high potassium (muscle weakness, an abnormal heartbeat or chest pains) are emergencies: call 999 or go to A&E.
- Swelling of the lips, mouth, tongue or throat, or fast or difficult breathing, may signal a serious allergic reaction (anaphylaxis) and needs a 999 call.
Cautions and interactions
- The NHS lists past allergic reactions to trimethoprim or other medicines, liver or kidney problems, iron deficiency or folate deficiency anaemia, porphyria and other blood disorders among the reasons it may not suit a person.
- Pregnancy and trying to conceive are circumstances where the NHS advises checking with a doctor first.
- The NHS says trimethoprim can be taken in pregnancy but is not usually recommended in early pregnancy, because it can lower folic acid levels.
- For someone trying to conceive who takes it daily, the NHS says the doctor may change the antibiotic or prescribe high-dose folic acid.
- Small amounts pass into breast milk and side effects in the baby are unlikely with short-term use, though a pharmacist, doctor, midwife or health visitor is the person to ask.
- Medicines named by the NHS as interacting include rifampicin, warfarin, acenocoumarol, digoxin, phenytoin, repaglinide, pioglitazone and methotrexate.
- The NHS says there is not enough information to know whether herbal remedies or supplements can be taken with trimethoprim.
- MedlinePlus asks people to tell the prescriber about allergies to trimethoprim or sulfa drugs and about all other medicines, vitamins and supplements.
- The NHS says normal eating and drinking can continue during treatment.
- The NHS says alcohol can be drunk during treatment, though it is best not to drink too much.
Interactions
Check interactions
Taking Trimethoprim with other medicines? Add them to the checker to see what the official sources say about each pair: the mechanism, general management and monitoring, never a bare "safe".
Interactions documented in the official sources cited on each card. The list covers the medicines represented on this site and is not exhaustive; how the interaction data is compiled.
No pair records for this medicine are in the sources currently represented by Anatomy Nexus. Absence from this list does not mean no interaction exists.
Same-class and related medicines
Same class: Urinary antibacterials
Medicines that share the class of Trimethoprim. They are separate medicines with their own licences, doses and interactions, not alternative names for Trimethoprim.
Related medicines
Medicines often discussed alongside Trimethoprim: used together, compared with it, or acting on the same problem by a different route.
Key facts
| Fact | Detail |
|---|---|
| Type | Antibiotic |
| Main use | Treating and preventing urinary tract infections such as cystitis |
| How it works | Interferes with folic acid production in bacteria |
| Folate caution | Not usually recommended in early pregnancy; low folate levels are a reason it may not suit |
| Urgent signs | Unexplained bruising or bleeding, sore throat, mouth ulcers or peeling skin: NHS 111 or a doctor promptly; swelling of the lips or throat, or difficulty breathing: 999 |
| Availability | On prescription in the UK |
What is trimethoprim used for?
Trimethoprim is an antibiotic, a medicine that treats infections caused by bacteria. The NHS says it is used to treat and to prevent urinary tract infections, such as cystitis. Less often it is used for other infections, including chest infections and acne. NICE guidance on lower urinary tract infection names trimethoprim as one of two first-choice antibiotics, alongside nitrofurantoin. NICE adds that resistance to trimethoprim is high across the country, so it is considered when the individual risk of resistance is low. NICE describes the risk of resistance as more likely to be lower when trimethoprim has not been used in the previous 3 months. In men, NICE notes that trimethoprim can reach useful levels in prostate tissue. Antibiotics do not work against colds, flu or other viral infections, as the NHS points out.
How does trimethoprim work, and why does folic acid come up?
Trimethoprim works by interfering with the way bacteria make folic acid. The NHS explains that a long course can occasionally lower folic acid levels in the body. Low folate levels appear on the NHS list of reasons why the medicine may not be suitable. The NHS also notes that the medicine can lower folic acid levels in pregnancy, which is why it is not usually recommended in early pregnancy. An NHS England patient safety case study states that taking trimethoprim in early pregnancy can result in spinal cord malformation in the foetus. For someone trying to conceive who takes the medicine daily, the NHS says the doctor may switch antibiotic or prescribe high-dose folic acid.
What are the side effects, and which need urgent help?
The NHS lists itching or a mild rash, feeling or being sick, diarrhoea, thrush and headaches as common effects. Being sick or having severe diarrhoea can stop contraceptive pills from working. Unexplained bruising or bleeding, a sore throat, mouth ulcers or a high temperature can signal a blood problem. Blistering or peeling skin is a serious skin reaction. Bloody or severe diarrhoea, severe stomach pain and yellowing of the skin or eyes also need prompt contact with NHS 111 or a doctor. Signs of meningitis (described by the NHS as very rare), high potassium (muscle weakness, an abnormal heartbeat or chest pains) or a serious allergic reaction, such as swelling of the lips, mouth, tongue or throat, are emergencies: call 999 or go to A&E.
Who needs extra care with trimethoprim?
The NHS lists past allergic reactions to trimethoprim or other medicines, liver or kidney problems, low folate levels, porphyria and other blood disorders as reasons the medicine may not suit someone. MedlinePlus also asks people to tell the prescriber about allergies to trimethoprim or sulfa drugs. Pregnancy is another consideration. The NHS says trimethoprim can be taken in pregnancy but is not usually recommended in the early weeks. It says there is no evidence that the medicine affects fertility in men or women. For someone trying to conceive who takes it daily, a doctor may switch antibiotic or prescribe high-dose folic acid. The NHS says small amounts pass into breast milk and that side effects in the baby are unlikely with short-term use.
Trimethoprim with other medicines, alcohol and food
The NHS names several medicines that interact with trimethoprim and can raise the chance of side effects. These include rifampicin, warfarin, acenocoumarol, digoxin, phenytoin, repaglinide, pioglitazone and methotrexate. For headaches, the NHS lists paracetamol and ibuprofen as suitable to take with it. For herbal remedies and supplements, the NHS says there is not enough information to know whether they can be taken with it. The NHS says alcohol can be drunk during treatment, though it is best not to drink too much. Normal eating and drinking can continue. For sickness, the NHS suggests taking the tablets with or after food. Anyone taking several medicines, or taking vitamins and supplements, is asked by MedlinePlus to tell the prescriber, who may need to adjust doses or watch for side effects.
Frequently asked questions
Can trimethoprim be taken during pregnancy?
The NHS says it can be taken in pregnancy but is not usually recommended in early pregnancy, because it can lower folic acid levels. Anyone who is pregnant or trying to conceive is advised by the NHS to check with a doctor first. A doctor may choose a different antibiotic or prescribe high-dose folic acid.
Why does folic acid matter with trimethoprim?
Trimethoprim interferes with folic acid production in bacteria, and a long course can occasionally lower folic acid levels in the body. Low folate levels are on the NHS list of reasons the medicine may not suit someone, and early pregnancy is a time of particular care.
What side effects need urgent attention?
Unexplained bruising or bleeding, sore throat, mouth ulcers, a high temperature, blistering or peeling skin, severe or bloody diarrhoea and yellow skin or eyes need prompt contact with NHS 111 or a doctor. Swelling of the lips, mouth, tongue or throat, fast or difficult breathing, muscle weakness or an abnormal heartbeat call for 999 or A&E.
Can alcohol be drunk while taking trimethoprim?
The NHS says alcohol can be drunk during treatment, but it is best not to drink too much. It also says normal eating and drinking can continue.
Does trimethoprim help with colds or flu?
No. The NHS notes that antibiotics do not work against colds, flu or other viral infections. Trimethoprim is mainly used to treat and prevent urinary tract infections, and sometimes chest infections and acne.
Connected pages in detail
What each linked page on Anatomy Nexus covers, in a line, for Trimethoprim.
Conditions
- Urinary tract infection Bacterial infection of the bladder (cystitis) or, when it ascends, the kidney (pyelonephritis), usually by gut bacteria such as Escherichia coli.
Symptoms
- Rash Any change in the colour or texture of the skin.
- Nausea The unpleasant sensation of being about to vomit.
- Vomiting Forceful emptying of the stomach through the mouth, a protective reflex run by the brainstem that closes the airway.
- Fever A body temperature of 38 °C (100.4 °F) or more, produced when the hypothalamus raises its set point in response to inflammatory signals.
Medical tests
- Urine culture A urine sample is spread on culture plates and incubated for 24-48 hours to grow any bacteria, identify them and test which antibiotics kill them.
- Full blood count The most commonly ordered blood test: an automated count of the red cells, white cells and platelets in a small sample.
- Vitamin B12 and folate Two blood tests, usually ordered together, for the vitamins the bone marrow needs to make normal red cells and the nervous system needs to keep…
Medications
- Amoxicillin A widely used penicillin antibiotic that kills bacteria by breaking their cell walls, prescribed for chest, ear, sinus.
Drug classes
- Urinary antibacterials Antibiotics used for urinary tract infections.