HIV protease inhibitor (used as pharmacokinetic booster) Pregnancy: Can be used during pregnancy if clinically needed — large exposure data (mostly at booster doses in combination therapy) show no increase in birth defects. Adversely interacts with oral contraceptives. Women living with HIV should not breast-feed.

Ritonavir

Brand names: Norvir

Ritonavir is an HIV protease inhibitor used almost exclusively at low dose as a pharmacokinetic booster to enhance levels of other protease inhibitors and some antivirals.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: As an antiretroviral agent: 600 mg twice daily (total 1,200 mg/day). Initiate at 300 mg twice daily for 3 days, then increase in 100 mg twice-daily increments up to 600 mg twice daily over no more than 14 days

Route: Oral (with food; swallow tablets whole)

Frequency: Twice daily

Max: 600 mg twice daily

Should be prescribed by physicians experienced in HIV treatment. As a pharmacokinetic enhancer (booster), the ritonavir dose depends on the co-administered protease inhibitor — consult that PI's SPC. Approved combinations include: atazanavir 300 mg once daily + ritonavir 100 mg once daily; darunavir 600 mg twice daily + ritonavir 100 mg twice daily (or darunavir 800 mg + ritonavir 100 mg once daily); lopinavir/ritonavir co-formulated 400/100 mg twice daily (or 800/200 mg once daily); saquinavir 1,000 mg twice daily + ritonavir 100 mg twice daily; fosamprenavir 700 mg + ritonavir 100 mg twice daily; amprenavir 600 mg + ritonavir 100 mg twice daily; tipranavir 500 mg twice daily + ritonavir 200 mg twice daily (not in treatment-naïve patients). Tablets must not be chewed, broken or crushed. Elderly: no dose adjustment needed. Paediatric (as an antiretroviral agent, children ≥2 years, eMC): 350 mg/m² twice daily, not exceeding 600 mg twice daily; start 250 mg/m² and increase by 50 mg/m² at 2–3 day intervals. This is a body-surface-area dose, not a per-kg dose; not recommended below 2 years. As a PK enhancer in children, refer to the co-administered PI's information. Verify paediatric dosing against a current children's formulary.

Dose adjustments

Renal

Renal clearance of ritonavir is negligible, so no reduction in total body clearance is expected and no specific dose adjustment is defined; monitor renal function if serious vomiting/diarrhoea occurs. Unlikely to be removed by haemodialysis or peritoneal dialysis. (Hepatic: must not be given in decompensated or severe hepatic impairment.)

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Hypersensitivity to ritonavir or any excipient
Decompensated liver disease
Potent CYP3A/CYP2D6 inhibitor — numerous co-medications are contraindicated, e.g. alfuzosin; pethidine and propoxyphene; ranolazine; neratinib and venetoclax; antiarrhythmics (amiodarone, bepridil, dronedarone, encainide, flecainide, propafenone, quinidine); fusidic acid; voriconazole (with ritonavir ≥400 mg twice daily); astemizole and terfenadine (list continues in SPC)

Side effects

Diarrhoea (may be severe with electrolyte imbalance)
Nausea and vomiting
Abdominal pain (upper and lower)
Oral and peripheral paraesthesia; dysgeusia; headache; dizziness
Fatigue/asthenia

Interactions

Potent inhibitor of CYP3A- and CYP2D6-mediated metabolism — markedly increases plasma concentrations of many co-administered drugs, with risk of serious/life-threatening reactions
Adversely interacts with oral contraceptives — use an alternative effective and safe method of contraception
Enzyme-modulating effect may be dose-dependent (some contraindications more relevant when ritonavir is used as an antiretroviral agent than as a booster, e.g. rifabutin, voriconazole)

Clinical monograph

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.

Reference: BHIVA; UK Liverpool HIV interactions; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways