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.
Ritonavir (brand name Norvir) is a protease inhibitor originally developed to treat HIV directly. Today it’s almost always used at a low dose as a pharmacokinetic booster — a drug that slows the breakdown of other HIV medicines in the liver, keeping their levels high enough to work effectively. You’ll see it paired with darunavir, atazanavir and other protease inhibitors.
Here’s how ritonavir works as a booster and what to expect if it’s part of your regimen.
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Ritonavir inhibits a liver enzyme called CYP3A4. This enzyme breaks down many drugs, including protease inhibitors. By blocking CYP3A4, ritonavir slows the breakdown of the main HIV drug, allowing it to stay in the blood at effective levels for longer. This means the main drug can be taken less often and at lower doses.
| Feature | Ritonavir | Cobicistat |
|---|---|---|
| Type | Protease inhibitor (at full dose) | Dedicated booster only |
| CYP3A4 inhibition | Yes | Yes |
| Anti-HIV activity at booster dose | Minimal | None |
| Available as standalone | Yes (Norvir) | Yes (Tybost) |
| Effect on creatinine | None | Raises creatinine slightly (not kidney damage) |
Common Drug Combinations
- Darunavir + ritonavir (most common in UK practice)
- Atazanavir + ritonavir
- Lopinavir/ritonavir (Kaletra — older combination, less used now)
At the 100mg booster dose, side effects are usually mild:
- Gastrointestinal symptoms — nausea, diarrhoea, taste changes
- Can raise cholesterol and triglycerides
- May affect blood sugar in some people
Drug Interactions
Because ritonavir is a powerful enzyme inhibitor, it affects many other drugs. Always tell your HIV team and pharmacist about everything you take, including:
- Statins (some contraindicated)
- Blood thinners
- Erectile dysfunction drugs (dose must be reduced)
- Steroids (especially inhaled fluticasone — can cause Cushing’s syndrome)
- Recreational drugs (MDMA and methamphetamine levels can be dangerously raised)
- Herbal medicines and supplements
Read more: Darunavir | What is HIV? | HIV and the immune system
Frequently Asked Questions
Rarely. At the full therapeutic dose, ritonavir has significant side effects. At the low 100mg booster dose, it’s well tolerated and is included to boost other drugs rather than to treat HIV itself.
Some statins are contraindicated with ritonavir. Pravastatin and rosuvastatin are the preferred options. Your HIV team will advise.
Yes. Ritonavir dramatically raises levels of MDMA, methamphetamine and some other recreational drugs, which can be life-threatening. Speak to your HIV team honestly about drug use.
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.
