Ritonavir

How it works

It potently inhibits the cytochrome P450 3A4 enzyme, slowing metabolism of co-administered drugs and thereby boosting their plasma concentrations.

Prescribing in practice

• As a strong CYP3A4 inhibitor, ritonavir causes extensive and potentially serious drug interactions; screen all concomitant medicines before and during treatment.
• Several commonly used drugs are contraindicated owing to risk of toxic accumulation, so check interactions meticulously.
• It is used predominantly as a booster rather than for its own antiviral effect.

Monitoring

Monitor for drug-interaction effects, liver function and lipids during therapy.

Counselling the patient

• Always tell any prescriber or pharmacist that you take ritonavir, as it interacts with many medicines.
• Do not start new medicines, including over-the-counter or herbal products, without checking first.
• Report any new or unusual side effects, which may indicate an interaction.

Evidence & guidelines

Low-dose ritonavir boosting is a well-established pharmacokinetic strategy in HIV and other antiviral therapy.