References UK clinical guidance from the NHS, the British HIV Association (BHIVA), and the British Association for Sexual Health and HIV (BASHH). Last updated: April 2026.
Septrin (co-trimoxazole) is a combination antibiotic used to prevent opportunistic infections in people living with HIV, particularly those with weakened immune systems. It’s not an antiretroviral drug and doesn’t treat HIV itself, but it significantly reduces the risk of serious infections like pneumocystis pneumonia (PCP), toxoplasmosis, and certain bacterial infections.
If you’ve been prescribed Septrin alongside your HIV treatment, this guide explains why it’s important and what to expect.
Why Is Septrin Prescribed for HIV?
People with HIV who have low CD4 counts (below 200 cells/mm3) are vulnerable to infections that a healthy immune system would normally fight off. These are called opportunistic infections. Septrin acts as prophylaxis, preventing these infections before they occur.
BHIVA guidelines recommend starting Septrin prophylaxis when the CD4 count drops below 200, and continuing until it rises above 200 for at least 3 months on antiretroviral therapy.
What Does Septrin Prevent?
| Infection | Risk Without Prophylaxis | Septrin Effectiveness |
|---|---|---|
| Pneumocystis pneumonia (PCP) | High when CD4 < 200 | Reduces risk by ~90% |
| Toxoplasmosis | Moderate when CD4 < 100 | Highly effective prevention |
| Bacterial infections | Increased with low CD4 | Reduces bacterial pneumonia and diarrhoea |
| Isosporiasis | Low but serious | Effective prevention |
How to Take Septrin
- Standard dose: 960mg (one double-strength tablet) once daily
- Take with or after food to reduce stomach upset
- Stay well hydrated (drink plenty of water)
- Continue as prescribed even if you feel well
Side Effects
- Nausea, loss of appetite (common, usually mild)
- Skin rash (can be serious — stop and seek advice if rash develops)
- Headache
- Blood count changes (monitored with regular blood tests)
- Sensitivity to sunlight
Important: Septrin can cause a serious allergic rash. If you develop a rash, fever, or mouth ulcers while taking Septrin, stop the medication and contact your HIV clinic immediately.
When Can You Stop Septrin?
You can usually stop Septrin once your CD4 count has been above 200 for at least 3 consecutive months on antiretroviral therapy. Your HIV specialist will make this decision based on your blood results.
Read more: What is HIV? | Starting HIV treatment | CD4 counts explained
Frequently Asked Questions
Is Septrin the same as HIV treatment?
No. Septrin is an antibiotic that prevents opportunistic infections. It does not treat HIV or reduce viral load. You still need antiretroviral therapy (ART) to control HIV.
Can I take Septrin if I’m allergic to sulfa drugs?
Co-trimoxazole contains sulfamethoxazole, a sulfa drug. If you have a sulfa allergy, your doctor may try a desensitisation protocol or prescribe an alternative prophylaxis (dapsone or atovaquone).
Does everyone with HIV need Septrin?
No. It’s only recommended for people with CD4 counts below 200 (or with a history of opportunistic infections). People with well-controlled HIV and normal CD4 counts don’t need it.
Related reading
- Choosing the Right PrEP Drug: Truvada vs Descovy
- Dovato for HIV: A Two-Drug Regimen
- Triumeq for HIV: A Straightforward Guide
- Efavirenz for HIV: An Older NNRTI Still in Use
- Eviplera: How This Anti-HIV Drug Works
Sources
- NHS — Sexual health conditions
- BASHH — UK clinical guidelines for STI management
- British HIV Association (BHIVA) — HIV treatment guidelines
- Terrence Higgins Trust — sexual health information
- UK Health Security Agency — STI surveillance data
Steve Page is a recognised expert on Sexually Transmitted Diseases (STDs) and STD treatments, having published numerous articles in peer-reviewed journals and presented his research at conferences around the world. He has an in-depth understanding of the latest medical research on STDs, and is an advocate for the development of new treatments and protocols to improve the health of those affected. In addition to his research, he has dedicated his career to understanding the causes and symptoms of STDs, as well as how to best treat those impacted.
