Nirmatrelvir/Ritonavir (Paxlovid)
Is It Right for You?
A complete guide to Nirmatrelvir/Ritonavir (Paxlovid) — what it treats, how it works, dosages, side effects, and when a clinician may prescribe it following an online consultation.
What Is Paxlovid (Nirmatrelvir/Ritonavir)?
Paxlovid is an oral antiviral treatment for COVID-19 consisting of two co-packaged components: nirmatrelvir — a SARS-CoV-2 main protease inhibitor — and ritonavir — a pharmacokinetic booster that slows the breakdown of nirmatrelvir, maintaining therapeutic drug levels. It is taken as three tablets (two nirmatrelvir 150 mg tablets and one ritonavir 100 mg tablet) twice daily for 5 days.
Paxlovid is indicated for adults who have confirmed COVID-19 infection and are at high risk of progression to severe disease, hospitalisation, or death. Clinical trial data (EPIC-HR) demonstrated that treatment initiated within 5 days of symptom onset reduced the risk of hospitalisation or death by approximately 89% in high-risk unvaccinated patients. Paxlovid must be started as early as possible after a positive COVID-19 test and symptom onset — and no later than 5 days after the first symptoms.
Paxlovid is a prescription-only medication in the UK and is only prescribed to patients who meet specific high-risk criteria defined by NICE and NHS England guidance. A licensed clinician must assess eligibility and current medications (due to significant drug interactions) before prescribing.
What Conditions Is Paxlovid Used For?
Paxlovid is indicated for:
treatment of symptomatic COVID-19 infection in adults who do not require supplemental oxygen but who are at high risk of disease progression to severe illness or death. High-risk criteria typically include: patients who are immunocompromised, have significant comorbidities (uncontrolled diabetes, severe renal or liver disease, severe cardiovascular or respiratory disease), or are elderly with multiple risk factors.
The clinical efficacy of Paxlovid has been demonstrated only when initiated within 5 days of first symptoms. Treatment started later has not been shown to be beneficial in most patient populations. If you develop COVID-19 symptoms and believe you are at high risk, contact your GP or NHS COVID medicines service immediately for rapid assessment.
How Does Paxlovid Work?
Nirmatrelvir is a peptidomimetic inhibitor of the SARS-CoV-2 main protease (Mpro, also called 3CLpro). This enzyme is essential for processing the large polyprotein precursors produced during viral replication into functional viral proteins. By blocking the main protease, nirmatrelvir prevents the maturation and functional assembly of new viral particles — halting viral replication within infected cells.
The main protease is highly conserved across coronaviruses and differs significantly from human proteases, providing high selectivity and a favourable safety profile. Resistance to nirmatrelvir can develop through mutations in Mpro, though clinically significant resistance in standard treatment courses has been uncommon.
Ritonavir is itself an HIV protease inhibitor but is given here at a sub-therapeutic dose purely as a pharmacokinetic booster. Ritonavir is a potent inhibitor of CYP3A4 — the primary enzyme responsible for metabolising nirmatrelvir. By inhibiting CYP3A4, ritonavir dramatically slows the breakdown of nirmatrelvir, maintaining therapeutic nirmatrelvir plasma concentrations throughout the 5-day course without requiring higher or more frequent doses of nirmatrelvir. This "boosting" strategy is well established in HIV medicine (where ritonavir-boosted protease inhibitors are a mainstay of therapy).
Dosages & Administration
The correct dose depends on the type and severity of infection, age, weight, and kidney function. Always follow your clinician's instructions. The table below is for general reference only.
Administration Tips
Take with or without food — nirmatrelvir/ritonavir can be taken regardless of meals.
Take tablets whole — do not crush or chew.
Complete the full 5-day course even if you feel significantly better — incomplete courses may allow viral replication to resume.
Separate from medications that interact — ritonavir has extensive drug interactions. Your clinician will have reviewed your medication list. Follow any specific timing instructions for other medications.
Start as early as possible after a positive test and symptom onset — efficacy is time-dependent and falls off if treatment is delayed beyond 5 days.
Side Effects of Paxlovid
Paxlovid is generally well tolerated over the 5-day course. Most side effects are mild and transient.
- Dysgeusia (metallic or altered taste) — very common; caused by ritonavir
- Diarrhoea
- Hypertension (transient)
- Myalgia (muscle aches)
- Nausea
- Headache
- Severe hypersensitivity reactions — rare; anaphylaxis, urticaria
- Drug interactions causing serious adverse events — the most clinically significant concern; ritonavir's CYP3A4 inhibition can raise co-medications to toxic levels (see interactions below)
- COVID-19 rebound — some patients experience a return of mild COVID-19 symptoms 7 to 14 days after completing Paxlovid; this appears to be viral rebound and is usually mild and self-limiting; isolation guidance should be followed
A small proportion of patients experience a recurrence of COVID-19 symptoms approximately 7 to 14 days after finishing their Paxlovid course. This 'Paxlovid rebound' appears to reflect incomplete viral suppression with re-expansion of residual virus. Current evidence suggests it is generally mild. Patients should follow current government isolation guidelines if symptoms recur and discuss with their clinician whether a second course of treatment is appropriate.
Drug Interactions
Ritonavir is one of the most potent CYP3A4 inhibitors in clinical use, and Paxlovid has one of the most extensive and clinically critical drug interaction profiles of any medication. A comprehensive medication review is mandatory before prescribing Paxlovid. Many commonly prescribed drugs require temporary dose adjustment, temporary substitution, or are absolute contraindications.
[CONTRAINDICATED — DO NOT CO-ADMINISTER]:
- Statins metabolised by CYP3A4: Simvastatin, Lovastatin (risk of fatal rhabdomyolysis) — must be stopped for the 5-day course
- Triazolam, Oral Midazolam (risk of extreme sedation)
- Ergotamine, Dihydroergotamine (risk of ergotism)
- Colchicine (in patients with renal or hepatic impairment — risk of colchicine toxicity)
- Amiodarone, Flecainide, Propafenone (risk of cardiac arrhythmia)
- Ranolazine, Ivabradine (cardiac risk)
- Lurasidone, Pimozide, Quetiapine (risk of serious arrhythmias)
- Rifampicin (reduces nirmatrelvir/ritonavir levels to sub-therapeutic)
- St John's Wort (reduces nirmatrelvir/ritonavir levels)
Levels dramatically increased; require dose reduction and intensive monitoring
INR altered; close monitoring required
minor interaction; continue with monitoring
Effectiveness may be reduced; use additional barrier contraception.
levels may be reduced; discuss with clinician
Important Warnings
Paxlovid is only appropriate after a thorough review of all co-medications. Ritonavir boosting creates clinically significant interactions with a very large number of drugs. Prescribing Paxlovid without a complete medication review risks serious or fatal drug toxicity.
Paxlovid requires dose adjustment in moderate renal impairment (eGFR 30–60 mL/min) and is not recommended in severe renal impairment (eGFR < 30 mL/min). Renal function must be assessed before prescribing.
Paxlovid is not recommended during pregnancy based on current data. Ritonavir is used in pregnancy in HIV management, but the combination at these doses for COVID-19 lacks sufficient safety data. Breastfeeding should be suspended during and for 7 days after completing Paxlovid. The risk-benefit of treatment versus untreated COVID-19 in pregnancy should be assessed by a specialist.
Speak to a Clinician About Treatment
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The treatment you need, when you need it.
Paxlovid FAQs
Paxlovid is available on the NHS to adults with confirmed COVID-19 who are at highest risk of severe disease or hospitalisation. Eligibility is determined by NICE and NHS England guidance and typically includes patients who are immunocompromised (e.g. on immunosuppressants, undergoing chemotherapy, with HIV, or after organ transplant), the elderly with significant comorbidities, or those with conditions such as severe renal or liver disease, Down syndrome, or multiple sclerosis. Your GP or local COVID medicines delivery unit can assess eligibility.
The altered or metallic taste (dysgeusia) is a very common and recognised side effect of ritonavir and affects the majority of patients. It is typically described as a persistent bitter or metallic taste that begins within the first day and resolves shortly after completing the 5-day course. It is unpleasant but harmless.
Some patients experience a recurrence of COVID-19 symptoms — typically mild — approximately 7 to 14 days after finishing their Paxlovid course. Current evidence suggests this reflects re-expansion of residual virus that was not fully cleared during the 5-day course. Rebound is usually mild, self-limiting, and does not appear to increase the risk of severe disease. Patients should follow isolation guidance and contact their clinician if a second course may be warranted.
It depends on which statin. Simvastatin and lovastatin are metabolised almost exclusively by CYP3A4, and ritonavir can raise their levels to potentially fatal concentrations — these must be stopped for the duration of Paxlovid. Atorvastatin can be continued at reduced doses (maximum 20 mg during the 5-day course). Rosuvastatin and pravastatin are not metabolised by CYP3A4 and can be continued without adjustment. Your clinician will advise on the appropriate management of your statin during Paxlovid treatment.