Norvir (ritonavir) in its standard 100 mg tablet form is used primarily as a pharmacokinetic booster for other HIV protease inhibitors, not as a standalone antiretroviral treatment. At low booster doses (100 mg once or twice daily), it blocks the liver enzyme (CYP3A4) that breaks down other protease inhibitors, keeping their blood levels higher and allowing them to be effective with smaller or less frequent doses.

It is commonly used to boost atazanavir (Reyataz), darunavir (Prezista), and lopinavir (Kaletra), among others.

When used as a booster, take 100 mg once or twice daily as directed by your HIV doctor, always with food. Fatty food significantly improves absorption of ritonavir.

Norvir is always used in combination with another antiretroviral. Never take it alone as the sole HIV medicine.

Take the missed dose with food as soon as you remember. If it is nearly time for the next dose, skip and continue your schedule. Never take a double dose. Contact your HIV doctor if you miss doses often.

Contact your HIV specialist or a poison control centre immediately.

• Tablets: store below 30°C
• Keep in the original container
• Keep out of reach of children

At booster doses (100 mg), side effects are generally milder than at the higher therapeutic doses used historically.

Common side effects at booster doses:

• Nausea
• Diarrhoea
• Abdominal discomfort
• Altered taste
• Raised triglycerides and cholesterol (less than at full doses)

Serious effects at higher doses:

• Pancreatitis
• Liver toxicity
• PR and QT interval changes (heart rhythm effects)

Blood tests for lipids and liver function should be checked regularly.

Ritonavir is one of the most potent drug interaction agents known in medicine. As a CYP3A4 inhibitor, it affects the levels of a very large number of medicines:

• Statins: ritonavir significantly affects how many statins are processed, which can raise the risk of serious muscle side effects. Your doctor will review which statins are safe to use alongside Norvir
• Sedatives and sleeping tablets (midazolam, triazolam): dramatically increased levels. Avoid
• Antiarrhythmics (amiodarone, flecainide): can cause dangerous heart rhythm problems
• Phosphodiesterase inhibitors (sildenafil, tadalafil): ritonavir significantly increases their blood levels, raising the risk of serious side effects. Tell your doctor you are on Norvir before these medicines are prescribed
• Opioid pain medicines: variable and complex interactions
• Rifampicin, St John's Wort: reduce ritonavir levels

Always have your full medicine list reviewed by your HIV pharmacist before making any changes.

Am I taking Norvir as my main HIV treatment?

At the standard 100 mg tablet dose, Norvir is almost always used as a booster rather than a treatment in its own right. At this dose, it is not potent enough to suppress HIV on its own. Its purpose is to keep the levels of your other HIV medicines high. If your prescription says 100 mg, it is almost certainly a booster dose being used alongside a protease inhibitor.

Why does Norvir interact with so many medicines?

Ritonavir is a very potent inhibitor of the CYP3A4 enzyme in the liver, which is responsible for metabolising a very large proportion of all medicines. By blocking this enzyme, ritonavir increases the levels of many other drugs taken at the same time. This is useful for boosting HIV medicines but can be dangerous when it affects other prescribed medicines. Your HIV pharmacist should review all your medicines carefully.

Ace provides accurate and independent information medically reviewed on prescription medications. This material is provided for educational purposes only and is not intended for medical advice, diagnosis or treatment.