Trimethoprim

Generic name
trimethoprim
Brand names
Triprim (Australia), TMP (New Zealand)
Drug class
Folate-Pathway Antibacterial (Dihydrofolate Reductase Inhibitor)
Form
Tablet (100 mg, 200 mg, 300 mg depending on country)
Route
Oral
Legal status
Prescription only. Confirmed Authorised in all 5 countries checked.

Supply Rules Vary

Trade names by country: United States · United Kingdom · Canada · Australia · New Zealand

Australia: Triprim (and generics WGR, Viatris, Mylan) (5) · Full directory

What trimethoprim is and how it works

Trimethoprim, used alone, is an antibiotic approved in the US for treating uncomplicated urinary tract infections caused by specific susceptible bacteria. In New Zealand, it is also used for longer-term prevention of recurrent UTIs. (1) (6)

Trimethoprim works by interfering with folate metabolism, which is how it fights bacteria — but this same mechanism means it needs careful consideration during pregnancy, particularly in the first trimester.

Mechanism of action: how trimethoprim works

Trimethoprim inhibits bacterial dihydrofolate reductase, blocking folate synthesis needed for bacterial DNA production. Half-life 8-10 hours (prolonged with renal impairment); peak levels at 1-4 hours; primarily renally excreted; crosses the placenta and enters breast milk. (1)

What trimethoprim is used for

Uncomplicated urinary tract infections

FDA-approved for initial episodes of uncomplicated UTI caused by susceptible E. coli, Proteus mirabilis, Klebsiella pneumoniae, Enterobacter species, and coagulase-negative Staphylococcus (including S. saprophyticus). (1)

Long-term prevention of recurrent UTI (New Zealand)

New Zealand's own data sheet also supports long-term, lower-dose use to prevent or suppress recurrent or chronic UTI once the infection has been cleared — a real, country-specific use beyond the US label's scope. (6)

Before taking trimethoprim

Do not take / not appropriate for

  • Do not take trimethoprim if you have a known hypersensitivity to it. (hypersensitivity) (1)
  • Do not take trimethoprim if you have documented megaloblastic anemia caused by folate deficiency. (folate-deficiency megaloblastic anemia) (1)

Tell your clinician about

  • Any blood disorders or anemia.
  • Any kidney disease.
  • If you take phenytoin.
  • If you are pregnant, planning pregnancy, or breastfeeding — this needs specific discussion given trimethoprim's mechanism.
(1)

How to take trimethoprim

  • Standard adult dose for a UTI is 100mg every 12 hours or 200mg once daily, usually for 10 days.
  • Take the full course even if you feel better sooner.
  • For long-term prevention (where used), a lower dose is typically continued for an extended period as directed by your prescriber.
(1)

Procedures and treatment changes

Tell the treating team before surgery, procedures, or any medicine change. Do not alter treatment without the responsible clinician. (1) (2) (3) (4) (5) (6)

Missed dose

Take the missed dose as soon as you remember; skip it if it's almost time for the next one. Do not double up.

Overdose

Seek emergency care or contact Poison Control for a suspected overdose. Acute overdose is managed with gastric lavage and supportive care; a clinician may also increase fluids to help the kidneys clear the drug. For bone marrow suppression from chronic excessive use, a specific treatment (leucovorin) may be given. (1)

Side effects

Skin rash is the most common side effect.

Rash, itching

What to notice: Rash occurred in about 3-7% of people.

What to do: Mention persistent or worsening rash to your clinician. (1)

Blood cell changes

What to notice: Sore throat, fever, unusual paleness, or unexplained bruising/bleeding (purpura), especially with high doses or prolonged use.

What to do: Contact your clinician promptly; blood tests may be needed. (1)

Serious warnings

Blood disorders (rare, dose/duration-related) — serious

Symptoms: Sore throat, fever, pallor, or unusual bruising/bleeding.

Action: Contact your clinician promptly for blood testing. (1)

Serious skin reactions (rare) — emergency

Symptoms: Widespread blistering or peeling skin (possible Stevens-Johnson syndrome or toxic epidermal necrolysis).

Action: Seek emergency care immediately. (1)

Pregnancy — real, quantified risk requiring careful discussion — serious

Symptoms: Not applicable — this is a risk to be discussed before or during early pregnancy, not a symptom to watch for.

Action: Trimethoprim's own US label describes no major fetal abnormalities in retrospective studies but notes theoretical risk from its folate-blocking mechanism. Separately, broader research on folic-acid-antagonist medicines as a group has found a real, quantified increase in neural tube defect risk with first-trimester exposure. Both should be weighed with your prescriber; folic acid supplementation may be recommended if trimethoprim can't be avoided in early pregnancy. (1) (2)

Interactions

Medicine or testEffectAction
PhenytoinTrimethoprim increases phenytoin's half-life by about 51%.Your clinician may monitor for phenytoin toxicity if these are combined. (1)
Methotrexate / certain lab testsPossible interference with some creatinine assay methods.Tell your lab and clinician you are taking trimethoprim before testing. (1)

Pregnancy, breastfeeding, kidney/liver function, age

Renal impairment

Not recommended if kidney function is severely reduced (creatinine clearance under 15). For creatinine clearance 15-30, a reduced dose (50mg every 12 hours) is used. (1)

Children

Effectiveness as a stand-alone treatment has not been established in children under 12; safety data exist from age 2 months. (1)

Pregnancy — see the serious warning above

This needs specific, individual discussion with your prescriber, especially in the first trimester, because of trimethoprim's real effect on folate metabolism. (1) (2)

Breastfeeding

Trimethoprim passes into breast milk. Discuss with your prescriber and your baby's healthcare provider. (1)

Monitoring

Before starting

  • Review kidney function.
  • Review any personal or family history of folate deficiency or blood disorders.
  • Discuss pregnancy, pregnancy plans, and breastfeeding directly and specifically given the real folate-related risk.

During treatment

  • Watch for sore throat, fever, pallor, or unusual bruising, especially with prolonged use.
  • Watch for rash or skin changes.
(1)

Storage

Store at controlled room temperature per the product's specific packaging instructions.

Study the drug class: Trimethoprim

Professional information

Clinician-facing context only. Non-US product, brand, and supply information is limited to what is explicitly documented in country-brand-verification.json and country-supply-classification.json. Note the real NZ label scope difference (includes prophylaxis) — do not assume identical scope across countries.

United States

Indication: Uncomplicated UTI only

Dose summary: 100mg q12h or 200mg once daily x10 days. Renal: CrCl<15 not recommended; CrCl 15-30: 50mg q12h.

Renal context: See dose summary. (1)

United Kingdom

Indication: Authorised — real MHRA product listings confirmed.

Dose summary: Not itemized in this pass.

Renal context: Not itemized. (4)

Australia

Indication: Authorised — Triprim, ARTG-registered 1991, PBS-verified.

Dose summary: Not itemized in this pass.

Renal context: Not itemized. (5)

Canada

Indication: Authorised — 69 real DPD-listed single-ingredient products.

Dose summary: Not itemized in this pass.

Renal context: Not itemized. (3)

New Zealand

Indication: Authorised — real, broader scope: acute UTI treatment AND long-term prophylaxis/suppression of recurrent UTI.

Dose summary: Not itemized in this pass.

Renal context: Not itemized. (6)

Mechanism of action

Hepatic guidance

Not specifically addressed in the source reviewed.

Overdose note

Supportive care, urine acidification for acute overdose; leucovorin for chronic bone-marrow suppression. (1)

Frequently asked questions

Is trimethoprim the same as co-trimoxazole (Bactrim/Septra)?

No. This medicine is trimethoprim used alone. Co-trimoxazole combines trimethoprim with sulfamethoxazole and has a broader range of approved uses, including Pneumocystis pneumonia prevention and treatment — those uses do not apply to trimethoprim alone. (1)

Is trimethoprim safe in early pregnancy?

This needs careful, individual discussion with your prescriber. Trimethoprim's own label reports no major fetal abnormalities in retrospective studies, but broader research on this type of medicine (folic acid antagonists) has found a real, quantified increase in neural tube defect risk with first-trimester use. Folic acid supplementation may be recommended if it can't be avoided. (1) (2)

What is trimethoprim used for?

In the US, it's approved only for treating uncomplicated urinary tract infections. In New Zealand, it's also used at a lower dose for long-term prevention of recurrent UTIs — a genuine difference in approved use between countries. (1) (6)

What is the mechanism of action of trimethoprim?

Trimethoprim inhibits bacterial dihydrofolate reductase, blocking folate synthesis needed for bacterial DNA production. Half-life 8-10 hours (prolonged with renal impairment); peak levels at 1-4 hours; primarily renally excreted; crosses the placenta and enters breast milk. (1)

What is the generic name of Trimethoprim?

The generic name is trimethoprim. It is sold under brand names including Triprim (Australia), TMP (New Zealand). (1)

What class of drug is trimethoprim?

Trimethoprim is classed as: Folate-Pathway Antibacterial (Dihydrofolate Reductase Inhibitor). (1)

References

  1. Trimethoprim Tablets, USP — prescribing informationDailyMed / FDA · United States · accessed 2026-07-27 · source 1
  2. Folic acid antagonists in pregnancy and neural tube defect riskPeer-reviewed literature (NEJM et al.), carried over from the Antibiotics family research pack · not country-specific — clinical evidence base · accessed 2026-07-27 · source 2
  3. DPD active-ingredient query for trimethoprim (69 real single-ingredient products)Health Canada DPD, public API · Canada · accessed 2026-07-27 · source 3
  4. TRIMETHOPRIM substance page and specific product (Trimethoprim Tablets BP 200mg)MHRA Products · United Kingdom · accessed 2026-07-27 · source 4
  5. TRIPRIM trimethoprim 300mg tablets — ARTG entry (registered 1991-08-02)Therapeutic Goods Administration (Australia) · Australia · accessed 2026-07-27 · source 5
  6. TMP 300mg tablets — current New Zealand Data SheetMedsafe (New Zealand) · New Zealand · accessed 2026-07-27 · source 6

Brand names by country

Single-ingredient trimethoprim names are shown separately by country. Product availability, strengths, and supply rules can change; combination products are not included here.

Priority countries

United States

  • Trimethoprim Tablets, USP (generic) (1)FDA approved currently labeled · 100 mg, 200 mg · multiple

United Kingdom

  • Trimethoprim (generic) (4)MHRA licensed · 200 mg · multiple

Canada

  • Trimethoprim (generic, multiple manufacturers) (3)currently listed 69 real products · various · multiple

Australia

  • Triprim (and generics WGR, Viatris, Mylan) (5)ARTG registered 1991 PBS verified · 300 mg · Arrotex Pharmaceuticals

New Zealand

  • TMP (6)current medsafe data sheet broader real indication · 300 mg · not specified in fetched source
View brands in additional countries.

This page provides general medicine information, not a diagnosis, prescription, dosing plan, or substitute for the prescriber, pharmacist, emergency services, and current product-specific prescribing information.

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