A long-acting sulfonamide antibiotic
Sulfadimethoxine
An older, long-acting sulfonamide antibiotic that is now rarely used in people in the UK.
What is Sulfadimethoxine?
Sulfadimethoxine is an older sulfonamide antibiotic that works against certain bacteria. It is long-acting, so it stays in the body longer than some other sulfonamides. It is now rarely used in people in the UK, having been largely replaced by newer antibiotics. Like other sulfonamides, it can cause serious skin reactions and blood problems, and it can lead to crystals forming in the urine, so plenty of fluids should be drunk. It must not be used by people with a sulfonamide allergy or the inherited condition G6PD deficiency.
Education and reference only. This is a plain-language guide to Sulfadimethoxine — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Sulfadimethoxine is a sulfonamide, one of the oldest groups of man-made antibiotics. Sulfonamides work by stopping bacteria making a substance they need to grow. Sulfadimethoxine is described as long-acting because it lasts longer in the body than some related medicines. In the UK it is now largely a historical medicine in human use, having been overtaken by antibiotics that are more effective and safer for most infections. Where sulfonamides are still used, it is for specific reasons under medical guidance.
How it works
Sulfadimethoxine blocks an early step that bacteria use to make folate, a building block they need to grow and multiply. Human cells get their folate from food and do not make it this way, so the medicine targets bacteria rather than the body's own cells. By stopping the bacteria from growing, it gives the immune system time to clear the infection. Because it is long-acting, it stays at working levels in the body for longer than some other sulfonamides, which is part of why it was once used.
Company & origin
Originated / developed by: Generic (long-established).
An older long-acting sulfonamide antibiotic that is now rarely used in human medicine in the UK, having been largely superseded by newer antibiotics.
Practical use
How to take Sulfadimethoxine
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Take it by mouth exactly as prescribed, with a full glass of water.
- Drink plenty of fluids throughout the day to reduce the risk of crystals forming in the urine.
- Finish the full course you are given, even if you feel better, to help clear the infection properly.
- Tell your prescriber straight away about any rash, blistering, mouth sores or fever, as these can signal a serious skin reaction.
- Tell your prescriber if you have a sulfonamide allergy or the inherited condition G6PD deficiency before starting.
Weighing it up
Advantages & disadvantages of Sulfadimethoxine
Advantages
- A long-acting sulfonamide that stays at working levels in the body for longer than some related medicines.
- Targets bacteria through a step the body's own cells do not use.
- A long-established medicine with decades of experience behind its use.
Disadvantages
- Can rarely cause very serious skin reactions such as Stevens-Johnson syndrome.
- Can cause blood problems and crystals in the urine, so fluids and monitoring matter.
- An older medicine that is rarely used in people in the UK, having been largely superseded.
Practical use
Good to know
The most important things to understand about sulfadimethoxine are the safety points shared by all sulfonamides. It can rarely cause very serious skin reactions, including Stevens-Johnson syndrome and toxic epidermal necrolysis, so any rash, blistering, peeling skin, mouth sores or fever needs urgent medical attention. It can also affect the blood, lowering blood cells, and can cause crystals to form in the urine that may lead to kidney stones, which is why drinking plenty of fluids is advised. It must not be used by anyone allergic to sulfonamides or with the inherited condition G6PD deficiency, in whom it can cause red blood cells to break down. It is an older medicine that is rarely used in people in the UK today.
Who should not take it / use with caution
- People who are allergic to sulfonamides must not take it.
- People with the inherited condition G6PD deficiency should not take it, as it can cause red blood cells to break down.
- It is generally avoided in late pregnancy and in young babies, and used with care in people with kidney or liver problems.
Monitoring
- Watching for any sign of a serious skin reaction, especially early in treatment.
- Checking blood counts and kidney function with longer use.
- Encouraging good fluid intake to reduce the risk of urinary crystals.
Side effects
- Nausea, loss of appetite or other stomach upset.
- Skin rash, which can rarely progress to serious reactions such as Stevens-Johnson syndrome.
- Blood problems, such as a fall in blood cells, and crystals forming in the urine.
Key interactions
- It can increase the effect of warfarin and some diabetes medicines, raising the risk of bleeding or low blood sugar.
- It may interact with methotrexate, increasing its toxic effects.
- Tell your prescriber about all your medicines, as sulfonamides interact with several of them.
Available as: Tablets taken by mouth.
Answers
Sulfadimethoxine: frequently asked questions
What is sulfadimethoxine?
It is an older, long-acting sulfonamide antibiotic that works against certain bacteria. It is now rarely used in people in the UK, having been largely replaced by newer antibiotics.
Why do I need to drink plenty of fluids?
Sulfonamides can cause crystals to form in the urine, which may lead to kidney stones, so drinking plenty of fluids helps reduce that risk.
What serious side effects should I watch for?
Seek urgent medical attention for any rash, blistering, peeling skin, mouth sores or fever, as sulfonamides can rarely cause very serious skin reactions.
Who should not take it?
It must not be taken by anyone allergic to sulfonamides or with the inherited condition G6PD deficiency, in whom it can cause red blood cells to break down.
Is it still commonly used?
No. In the UK it is largely a historical medicine in human use, having been overtaken by antibiotics that are more effective and safer for most infections.
Authoritative sources
- BNF
- NICE CKS
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