Ritonavir Drug Profile

What is Ritonavir?

Ritonavir is an antiretroviral medication belonging to the protease inhibitor class of HIV drugs. It was originally developed to treat HIV infection but is now primarily used as a “booster” drug to enhance the effectiveness of other medications. Ritonavir gained widespread attention during the COVID-19 pandemic as a component of Paxlovid, an oral treatment for COVID-19.

Uses

• HIV treatment: Used in combination with other antiretroviral drugs to suppress HIV viral load and prevent progression to AIDS
• Pharmacokinetic enhancer: Low doses used to boost blood levels of other protease inhibitors and medications by inhibiting their breakdown
• COVID-19 treatment: Combined with nirmatrelvir in Paxlovid to treat mild-to-moderate COVID-19 in high-risk patients
• Hepatitis C treatment support: Sometimes used to enhance the effectiveness of certain hepatitis C medications

How it works

Ritonavir works by blocking protease enzymes that viruses need to replicate and mature. In HIV treatment, it prevents the virus from producing functional proteins necessary for creating new viral particles. When used as a booster, ritonavir inhibits cytochrome P450 enzymes in the liver, which normally break down other medications, allowing those drugs to remain active in the body longer at higher concentrations.

Side effects

Common side effects:

• Nausea and vomiting
• Diarrhea
• Stomach pain and indigestion
• Fatigue and weakness
• Metallic taste in mouth
• Headache
• Dizziness
• Loss of appetite

Serious side effects to watch for:

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• Severe liver problems (yellowing of skin or eyes, dark urine)
• Heart rhythm abnormalities (irregular heartbeat, fainting)
• Severe allergic reactions (rash, swelling, difficulty breathing)
• Pancreatitis (severe abdominal pain, nausea, vomiting)
• Severe skin reactions (Stevens-Johnson syndrome)

Warnings and precautions

• Liver disease: Use with extreme caution in patients with existing liver problems, as ritonavir can cause or worsen liver damage
• Heart conditions: May cause dangerous heart rhythm changes, especially in patients with existing heart disease or electrolyte imbalances
• Pregnancy: Generally considered safe during pregnancy for HIV treatment when benefits outweigh risks, but should only be used when clearly needed
• Drug resistance: Must be used as part of combination therapy to prevent development of resistant viral strains
• Blood sugar control: Can affect glucose levels and may worsen diabetes or cause new-onset diabetes

Interactions

Ritonavir has extensive drug interactions due to its effects on liver enzymes:

• Blood thinners (warfarin): May increase bleeding risk by affecting warfarin metabolism
• Statins (simvastatin, lovastatin): Can cause dangerous muscle damage by dramatically increasing statin levels
• Sedatives (midazolam, triazolam): May cause excessive sedation and breathing problems
• Heart medications (amiodarone, flecainide): Risk of life-threatening heart rhythm abnormalities
• Seizure medications (carbamazepine, phenytoin): Can reduce effectiveness of both medications
• Antifungals (ketoconazole): Mutual interaction increases levels of both drugs
• Birth control pills: May reduce contraceptive effectiveness
• St. John’s wort: Significantly reduces ritonavir levels and effectiveness

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Dosage

Ritonavir dosage varies significantly depending on its intended use. For HIV treatment, typical doses range from 600mg twice daily when used as a primary treatment, down to 100-200mg daily when used as a booster for other protease inhibitors. In COVID-19 treatment (Paxlovid), ritonavir is dosed at 100mg twice daily for five days in combination with nirmatrelvir. Dosage adjustments may be necessary in patients with liver impairment. All dosing should be individualized and determined by a healthcare provider based on the specific condition being treated, other medications being used, and patient factors.

Sources: FDA DailyMed drug labels (public domain), BNF, WHO Essential Medicines List. This article is for informational purposes only and does not constitute medical advice. Always consult your healthcare provider before starting or stopping any medication.

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