Can I take 7.5mg and Septra ds together?
Drug interaction guide
Originally published 14 Mar 2026
Meets Patient’s editorial guidelines
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Taking these two medicines together can cause the levels of methotrexate in your blood to rise to dangerous levels. This significantly increases the risk of severe side effects, including a drop in your blood cell counts (which helps you fight infection and stop bleeding) and damage to your kidneys or liver. In some cases, this combination can be life-threatening.
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Why this happens
Septra DS (co-trimoxazole) interferes with how your body handles methotrexate in two ways. First, it stops the kidneys from clearing methotrexate out of your system. Second, both drugs work by blocking 'folate' in the body; when used together, they starve your cells of this essential nutrient too much, leading to toxic effects.
What you should do
Back to contentsDo not take these two medicines together. You must contact your doctor or pharmacist immediately. They will likely need to prescribe a different antibiotic that does not interact with methotrexate. If you have already taken both, watch for signs of illness such as a sore throat, fever, unusual bruising, or mouth ulcers, and seek medical help right away.
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Using other medicines
Back to contentsTaking multiple medicines? Our Medicines Interaction Checker helps you check whether your prescriptions, over-the-counter medicines, and supplements are safe to take together. Simply search for your medicines to see potential interactions and what to do about them.
Disclaimer
This information is for general educational purposes only and should not be relied upon as a substitute for professional medical advice. Always consult your GP, pharmacist, or another qualified healthcare professional before making decisions about your medications. Individual circumstances may vary, and only a healthcare professional who knows your medical history can provide personalised guidance.
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Article history
The information on this page is written and peer reviewed by qualified clinicians.
14 Mar 2026 | Originally published

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