Antrin

Indications

• Urinary tract infections, such as cystitis (bladder infection).
• Prevention of repeated urinary tract infections, as a low dose taken long term.
• Some chest infections caused by sensitive bacteria.

It is also combined with Sulfamethoxazole for a wider range of infections. It does not work against viral infections.

How It Works

Trimethoprim is an antibiotic that blocks an enzyme bacteria need to make folic acid. Without folic acid the bacteria cannot make new genetic material, so they stop multiplying and the body can clear the infection.

Dosage and Administration

• Urinary tract infection (adults): usually 200 mg twice a day for 3 days in women with simple cystitis, or 7 days in men and in more complicated cases, as prescribed.
• Prevention of repeated infections (adults): usually 100 mg once a day at bedtime.
• Children: dose by body weight, as prescribed.

Take with or without food, with a glass of water. Finish the full course even if you feel better.

Interactions

• Methotrexate and Pyrimethamine: higher risk of blood problems due to lack of folic acid.
• Medicines that raise potassium, such as Enalapril, Losartan, Spironolactone and potassium supplements: potassium can rise too high.
• It can increase levels of Phenytoin and Digoxin, and the effect of Warfarin.
• Ciclosporin: higher risk of kidney problems.

Contraindications

• Allergy to trimethoprim.
• Anaemia caused by lack of folic acid (megaloblastic anaemia).
• Serious blood disorders.
• Severe kidney or liver disease where blood levels cannot be monitored.

Side Effects

Common: nausea, vomiting, itching, rash, headache and raised blood potassium.

Get urgent medical help for a widespread rash with blisters, peeling skin or mouth sores; fever, sore throat, mouth ulcers, unusual bruising or bleeding (possible blood problems); severe or bloody diarrhoea; muscle weakness or an irregular heartbeat (possible high potassium); or swelling of the face and difficulty breathing.

Pregnancy and Lactation

Trimethoprim is usually avoided in the first 3 months of pregnancy because it works against folic acid, which the baby needs. If a doctor decides it is necessary, extra Folic Acid may be given. It passes into breast milk in small amounts; short courses are generally considered acceptable while breastfeeding.

Precautions and Warnings

• Older people, people with poor nutrition, and those on long-term treatment are more likely to develop folic acid deficiency; blood tests may be needed.
• The dose may need lowering in kidney disease.
• Tell your doctor if you have porphyria.
• See a doctor if urinary symptoms do not improve within a few days.

Overdose

Too much can cause nausea, vomiting, dizziness, headache and confusion. Long-term overdose can affect the bone marrow. Seek medical help; doctors may give Folinic Acid.

Therapeutic Class

Antibacterial (folate antagonist).

Storage

Store below 30°C in a dry place, away from light and out of the reach of children.