Ritonavir
1 medicine
Ritonavir is an antiviral used at low doses to boost other HIV and COVID-19 medicines by blocking the liver enzyme that breaks them down. This creates extensive drug interactions, and combining it with many common medicines is dangerous or contraindicated.
Key facts
- Ritonavir (found in Kaletra and Paxlovid) is a protease inhibitor, but at the low doses used today it mainly works as a booster for other antiviral drugs.
- It blocks a liver enzyme, CYP3A4, that breaks down many other medicines, so it raises and prolongs the levels of the drug it is paired with.
- This enzyme-blocking effect causes extensive drug interactions. Many common medicines, including some heart, cholesterol, sedative, and migraine drugs, must not be combined with ritonavir.
- Seek urgent care for yellowing of the skin or eyes, or an irregular heartbeat.
What Ritonavir treats
Ritonavir is used, in low doses, to boost blood levels of other protease inhibitors used against HIV, and to boost the antiviral nirmatrelvir in short courses for COVID-19. It is not used alone to treat HIV or COVID-19; its role is to raise and sustain the levels of the partner drug.
How Ritonavir works
Ritonavir blocks CYP3A4, a liver enzyme that normally breaks down many drugs, including other protease inhibitors. With that enzyme inhibited, the partner drug clears more slowly and stays at effective levels longer, allowing lower doses or fewer daily doses of the main treatment.
Before you take it
- Before starting ritonavir, your prescriber must review every medicine, supplement, and herbal product you take. Many common drugs are contraindicated because ritonavir raises their levels to dangerous concentrations.
- Certain heart rhythm drugs, some cholesterol-lowering statins, several sedatives, ergot migraine medicines, and herbal products such as St John's wort must not be combined with ritonavir.
- Tell your prescriber about liver disease, since ritonavir is processed by the liver and liver problems can raise drug levels further.
- This interaction risk applies even to short courses, such as the ritonavir-boosted COVID-19 antiviral.
Side effects
Common effects include nausea, diarrhoea, altered taste, and tingling around the mouth.
Seek urgent care for:
- Yellowing of the skin or eyes, dark urine, or severe fatigue, which can signal liver injury.
- A fast, slow, or irregular heartbeat.
- Signs of a serious allergic reaction: facial swelling, rash, or difficulty breathing.
Safety essentials
- Ritonavir's defining risk is its drug interactions. Because it blocks CYP3A4 so strongly, combining it with certain heart, cholesterol, sedative, or migraine medicines can raise them to toxic levels; always check every medicine against an interaction list before starting or stopping ritonavir.
- Never start or stop other medicines without checking with your prescriber or pharmacist first, since ritonavir's effect on drug levels can persist for days after the last dose.
- Regular liver function monitoring is recommended, especially with existing liver disease.
This page is educational and does not replace advice from a doctor or pharmacist who knows your health history.