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.
Cobicistat (brand name Tybost) is a pharmacokinetic (PK) booster used in combination with certain HIV drugs. It doesn’t treat HIV directly. Instead, it inhibits the liver enzyme that breaks down other HIV drugs, keeping their blood levels high enough to work with once-daily dosing.
Cobicistat is found in combination tablets like Stribild, Genvoya, Symtuza, Prezcobix, and Evotaz.
How Does Cobicistat Work?
Some HIV drugs (particularly protease inhibitors and elvitegravir) are broken down quickly by the CYP3A4 enzyme in the liver. Without a booster, you’d need to take these drugs multiple times a day to maintain effective levels. Cobicistat blocks CYP3A4, slowing the breakdown of the other drug so it stays in your system longer.
This is similar to how low-dose ritonavir has traditionally been used to boost protease inhibitors. Cobicistat was developed as a cleaner booster with fewer side effects than ritonavir.
Drugs Boosted by Cobicistat
- Darunavir (protease inhibitor) — in Prezcobix or Symtuza
- Atazanavir (protease inhibitor) — in Evotaz
- Elvitegravir (integrase inhibitor) — in Stribild and Genvoya
Side Effects
Cobicistat is generally well tolerated:
- Small increase in serum creatinine (doesn’t indicate actual kidney damage, but your doctor will monitor kidney function)
- Jaundice when combined with atazanavir (cosmetic, not harmful)
- Gastrointestinal symptoms
Drug Interactions
Because cobicistat inhibits CYP3A4, it can affect how other medications are metabolised. Important interactions include:
- Statins (simvastatin contraindicated)
- Erectile dysfunction drugs (dose reduction needed)
- Hormonal contraception
- Corticosteroids
- Antacids and proton pump inhibitors
Always tell your HIV specialist about all medications you take.
Read more: What is HIV? | HIV drug interactions | Starting HIV treatment
Frequently Asked Questions
Does cobicistat cause kidney damage?
No. Cobicistat inhibits a transporter in the kidneys, causing a small measured rise in serum creatinine. This looks like kidney function is declining, but actual kidney function (glomerular filtration rate) is unchanged. Your doctor understands this and will monitor appropriately.
Cobicistat was designed as a cleaner booster with fewer gastrointestinal side effects than ritonavir. However, both are effective. The choice depends on which combination regimen is being used.
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.
