Description
How it works
Trimethoprim is an antibiotic that works by:
- Blocking an enzyme (dihydrofolate reductase) that bacteria need to make folic acid
- Stopping bacterial growth and multiplication, so the infection gradually clears as your immune system removes the bacteria
It is active against many of the common bacteria that cause urinary tract infections, such as E. coli, as well as some respiratory and other infections when prescribed for those conditions.
Trimethoprim only works on bacterial infections – it does not treat viral illnesses such as colds or flu.
Who it may be suitable for
Trimethoprim 50 mg/5 ml Suspension may be prescribed for:
- Adults and children with urinary tract infections (UTIs)
- Adults and children with other infections caused by trimethoprim-sensitive bacteria (for example some chest or ear infections, if specifically prescribed)
- Long-term prevention of recurrent UTIs in adults and children, where a clinician has recommended prophylaxis
It is particularly useful for patients who:
- Have difficulty swallowing tablets (children, older adults, or those with swallowing problems)
- Need weight-based dosing, especially younger children
Who should not take Trimethoprim Suspension
You must not take this medicine if you:
- Are allergic to trimethoprim or any of the ingredients in the suspension
- Have severe liver disease
- Have megaloblastic anaemia or certain other serious blood disorders caused by folate deficiency
- Are pregnant
- Are a premature infant or a baby under 4 months of age
- Have rare hereditary fructose intolerance, as the suspension contains sorbitol
Trimethoprim is not related to penicillin, so it can usually be used in people with penicillin allergy, provided there is no allergy to trimethoprim itself.
Take special care and discuss with a clinician if you:
Tell the prescriber if you:
- Have kidney problems or are on dialysis
- Are elderly, especially with kidney or liver problems
- Have conditions linked to low folate levels (e.g. poor diet, certain medicines, alcoholism)
- Have a history of blood disorders or are taking medicines that affect the bone marrow (e.g. methotrexate, azathioprine)
- Have poorly controlled diabetes
- Take medicines that can raise potassium levels (e.g. ACE inhibitors, ARBs, potassium-sparing diuretics, potassium supplements, some heparin products)
- Are taking warfarin or other blood thinners, digoxin, phenytoin, repaglinide, cyclosporin, or certain chemotherapy or anti-malaria medicines
Extra blood tests (for kidney function, blood counts and potassium) may be needed, especially for long-term treatment or in higher-risk patients.
Side Effects
Not everyone gets side effects. When they do occur, they are often mild and short-lived, but some can be serious and need urgent attention.
Very common / common side effects include:
- Headache
- Nausea, vomiting or diarrhoea
- Stomach discomfort
- Skin rash or hives (urticaria)
- Fungal overgrowth (thrush) in the mouth or elsewhere
- High potassium levels in the blood (hyperkalaemia) – usually only picked up on a blood test, but severe cases can cause muscle weakness, irregular heartbeat or palpitations
Less common / rare side effects (seek medical advice if these occur):
- Loss of appetite
- Dizziness, tiredness or sleep problems
- Tingling or numbness
- Sore mouth, glossitis or stomatitis
- Mild changes in liver tests
Serious side effects – stop the medicine and get urgent medical help if you notice:
Severe allergic reaction
- Sudden wheezing, difficulty breathing or shortness of breath
- Swelling of the face, lips, tongue or throat
- Severe rash, blistering or peeling skin
- High fever, joint pains or feeling very unwell
These may be signs of anaphylaxis or serious skin reactions such as Stevens–Johnson syndrome (SJS) or toxic epidermal necrolysis (TEN).
Serious blood disorders
- Unusual bruising, bleeding, nosebleeds or purple spots on the skin
- Persistent sore throat, mouth ulcers or infections
- Unexplained tiredness, paleness or shortness of breath
These can be signs of bone marrow depression or other serious blood problems.
Liver problems
- Yellowing of the skin or whites of the eyes (jaundice)
- Dark urine, pale stools
- Persistent nausea, vomiting or abdominal pain
Severe skin reactions or vasculitis
- Widespread red or purple rash, often with blisters
- Painful red lumps under the skin (erythema nodosum)
Kidney problems or electrolyte imbalance
- Dramatic reduction in urine output, swelling of legs/ankles, or blood in the urine
- Symptoms of high potassium (e.g. palpitations, muscle weakness)
If you get any worrying symptoms, contact a doctor or urgent care service immediately.
Patient Information Leaflet
Please see HERE for the Patient Information Leaflet
Additional Info
Key facts about Trimethoprim 50 mg/5 ml Oral Suspension:
- Active ingredient: Trimethoprim 50 mg in each 5 ml
- Form: White, viscous oral suspension in a 100 ml glass bottle
- Use: Treatment and prevention of urinary tract infections and other susceptible infections
- Legal category: Prescription-only antibiotic
- Contains sorbitol – do not use if you have hereditary fructose intolerance
- Also contains methyl and propyl hydroxybenzoate (parabens), which can rarely cause allergic reactions
- Storage: Keep in the original bottle and outer carton, away from children. Check the label for any specific storage instructions and expiry date.
Pregnancy and breastfeeding
- Pregnancy: Trimethoprim suspension is contraindicated in pregnancy because it can affect folate metabolism, which is important for normal fetal development.
- Breastfeeding: Trimethoprim passes into breast milk. It must not be used if the breast-fed infant is under 4 months old. In older infants, risks and benefits must be weighed by a clinician.
Important interactions
Trimethoprim can interact with several medicines. Examples include:
- Folate antagonists / anticonvulsants – e.g. methotrexate, some anti-epileptics (increased risk of folate deficiency / blood problems)
- Bone marrow-suppressing drugs – e.g. azathioprine, mercaptopurine, cytotoxic chemotherapy
- Warfarin and other coumarin-type anticoagulants – increased risk of bleeding
- Digoxin and phenytoin – levels may increase; monitoring often needed
- Repaglinide – increased risk of low blood sugar; monitor blood glucose
- ACE inhibitors, ARBs, potassium-sparing diuretics, potassium supplements, potassium-containing salt substitutes, heparin – increased risk of high potassium
- Rifampicin – may reduce trimethoprim levels
- Dapsone – levels of both drugs may increase
Always tell your prescriber and pharmacist about all other medicines, including over-the-counter and herbal products.
Directions
Trimethoprim suspension is for oral use only.
- Always shake the bottle well before each dose
- Measure doses using the oral syringe or medicine spoon provided – do not use a kitchen teaspoon
- It can be taken with or without food; taking it with food may help if it upsets your stomach
- Take it exactly as prescribed by the clinician – dose and length of treatment depend on your age, weight and type of infection
Typical dosing regimens (for guidance only – follow your own prescription):
Adults and children over 12 years
- Treatment of UTIs and other susceptible infections:
- 200 mg (20 ml) twice daily
- Long-term prevention of recurrent UTIs:
- 100 mg (10 ml) once at night
Children 6 weeks to 12 years (treatment of UTIs) – based on 8 mg/kg per day, usually split into two doses:
- 6 weeks – 5 months: 25 mg (2.5 ml) twice daily
- 6 months – 5 years: 50 mg (5 ml) twice daily
- 6 – 12 years: 100 mg (10 ml) twice daily
Children 6 months to 12 years (prevention of recurrent UTIs) – based on 2 mg/kg once daily at night:
- 6 months – 5 years: 25 mg (2.5 ml) at night
- 6 – 12 years: 50 mg (5 ml) at night
Kidney problems
-
Doses may be reduced if kidney function is reduced; this will be determined by the prescriber.
Course length
- For acute infections (such as a simple UTI), treatment is often for 3–14 days, depending on the infection and clinical advice
- For prevention of recurrent UTIs, trimethoprim may be used long term, but always under regular medical review
Important:
- Always complete the prescribed course, even if you start to feel better – stopping early can allow infection to come back and encourage antibiotic resistance
- Do not share this medicine with anyone else
Missed a dose?
If you forget a dose:
- Take it as soon as you remember unless it is almost time for your next dose
- If it is nearly time for the next dose, skip the missed dose and take the next one at the usual time
- Do not take two doses at the same time to make up for a missed dose
For night-time preventive doses, if you forget a dose and remember the next day, skip the missed dose and continue as normal that night (unless advised otherwise by your prescriber).
If you miss multiple doses, or are on long-term prophylaxis, ask your clinician or pharmacist how to continue.
If you vomit shortly after a dose:
- If you are sick within about 1 hour of taking it, the dose may not have been fully absorbed – speak to your pharmacist or clinician; they may advise another dose depending on the situation
FAQs
What is Trimethoprim 50 mg/5 ml Suspension used for?
Trimethoprim 50 mg/5 ml Suspension is an oral antibiotic used to treat and prevent urinary tract infections and other sensitive bacterial infections, particularly when a liquid format is preferred for children or adults who have difficulty swallowing tablets.
How long will it take to work?
Most users notice symptom relief within 24 to 48 hours, but the full course must be completed to ensure the infection is cleared; you should seek medical advice if symptoms do not improve after 3 days or if you develop a high fever or flank pain.
Does this medicine treat all types of urinary infections?
This medicine treats many uncomplicated UTIs like cystitis if the bacteria are sensitive, though local resistance may require a urine test to confirm suitability, and more severe cases like kidney infections may require different antibiotics.
Is trimethoprim a type of penicillin?
No, trimethoprim is not a type of penicillin and is generally safe for those with a penicillin allergy, provided they have no known allergy to trimethoprim itself or the other ingredients in the suspension.
Can I drink alcohol while taking Trimethoprim Suspension
There is no direct interaction with alcohol, but drinking is best avoided or kept in moderation as it can worsen side effects like dizziness and stomach upset, and may irritate the bladder during a UTI.
Does it affect the contraceptive pill?
Trimethoprim does not typically interfere with the contraceptive pill, but severe vomiting or diarrhoea can reduce pill absorption, necessitating the use of extra protection like condoms and following missed pill protocols.
Can children take this medicine?
This medicine can be prescribed for children from 6 weeks of age, though it is not for premature infants or babies under 4 months; it must be dosed carefully according to weight and age as instructed by the prescriber.
What if my symptoms don’t improve or keep coming back?
If symptoms persist after 3 days or if you experience vomiting, fever, or recurrent infections, you should consult a clinician for further tests or a revised treatment plan.
What are signs I should seek urgent medical help?
Seek urgent medical help from 24 HR Pharmacy or emergency services for signs of sepsis, high fever, difficulty breathing, facial swelling, jaundice, unusual bleeding, or significant changes in urine output.
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