PAXLOVID (COPACKAGED)
Clinical safety rating: caution
Comprehensive clinical and safety monograph for PAXLOVID (COPACKAGED) (PAXLOVID (COPACKAGED)).
PAXLOVID (nirmatrelvir/ritonavir) is an antiviral combination: nirmatrelvir is a SARS-CoV-2 main protease (Mpro) inhibitor, preventing viral replication; ritonavir is a CYP3A inhibitor that boosts nirmatrelvir plasma levels.
| Metabolism | Nirmatrelvir: primarily metabolized by CYP3A4. Ritonavir: extensively metabolized by CYP3A4, and is a strong CYP3A inhibitor. |
| Excretion | Rental excretion of nirmatrelvir is the primary route: approximately 70% of nirmatrelvir dose is excreted unchanged in urine, and about 30% is eliminated via feces (biliary/fecal). Ritonavir is primarily metabolized and eliminated in feces (86%) and urine (11%). |
| Half-life | Nirmatrelvir terminal half-life is approximately 6 hours; ritonavir half-life is about 3–5 hours. Clinical context: Twice-daily dosing maintains therapeutic concentrations for SARS-CoV-2 treatment. |
| Protein binding | Nirmatrelvir: approximately 50% bound to plasma proteins (mainly albumin). Ritonavir: 98–99% bound, primarily to albumin and alpha-1-acid glycoprotein. |
| Volume of Distribution | Nirmatrelvir: Vd/F ~ 108 L (approx. 1.3 L/kg for a 80 kg individual). Ritonavir: Vd/F ~ 0.4 L/kg. Indicates extensive tissue distribution for nirmatrelvir. |
| Bioavailability | Oral: Nirmatrelvir alone absorption is poor; coadministration with ritonavir (as a pharmacokinetic booster) increases nirmatrelvir bioavailability to approximately 75% (based on AUC comparison to IV data). Ritonavir oral bioavailability is about 60-80%. |
| Onset of Action | Oral: Time to peak plasma concentration is 1.5–2 hours for nirmatrelvir (with ritonavir boosting). Clinical antiviral effect is observed within 24 hours of first dose. |
| Duration of Action | Antiviral effect persists for the dosing interval (12 hours) with twice-daily administration. Full 5-day course recommended to reduce viral rebound. |
| Molecular Weight | Nirmatrelvir: 499.5 Da; Ritonavir: 720.9 Da |
Nirmatrelvir 300 mg (two 150 mg tablets) plus ritonavir 100 mg (one 100 mg capsule) orally twice daily for 5 days, initiated as soon as possible and within 5 days of symptom onset.
| Dosage form | TABLET |
| Renal impairment | eGFR ≥60 mL/min: no adjustment. eGFR 30-59 mL/min: nirmatrelvir 150 mg and ritonavir 100 mg orally twice daily for 5 days. eGFR <30 mL/min: not recommended (no dosing recommendations available). |
| Liver impairment | Child-Pugh Class A (mild): no adjustment. Child-Pugh Class B (moderate): nirmatrelvir 150 mg and ritonavir 100 mg orally twice daily for 5 days. Child-Pugh Class C (severe): not recommended. |
| Pediatric use | Authorized for patients aged 12 years and older weighing at least 40 kg: nirmatrelvir 300 mg plus ritonavir 100 mg orally twice daily for 5 days. For patients <12 years or <40 kg: no authorized dosing; use only if prescribed by licensed healthcare provider under EUA with specific weight-based dosing: weight 40 kg or more: same as adult; weight 30-39 kg: nirmatrelvir 225 mg (one 150 mg tablet and one 75 mg tablet) plus ritonavir 100 mg orally twice daily for 5 days; weight 20-29 kg: nirmatrelvir 150 mg (one 150 mg tablet) plus ritonavir 100 mg orally twice daily for 5 days; weight less than 20 kg: no dosing recommended. |
| Geriatric use |
| 1st trimester | Limited human data; animal studies show no evidence of harm. Use only if potential benefit justifies risk. |
| 2nd trimester | Limited human data; no known teratogenic risk. Use only if potential benefit justifies risk. |
| 3rd trimester | Limited human data; no known fetal adverse effects near term. Use only if potential benefit justifies risk. |
Clinical note
Comprehensive clinical and safety monograph for PAXLOVID (COPACKAGED) (PAXLOVID (COPACKAGED)).
| Placental transfer | Nirmatrelvir and ritonavir are expected to cross the placenta due to low molecular weight and lipophilicity; no specific data. |
| Breastfeeding | No data on presence in human milk; nirmatrelvir and ritonavir are expected to be excreted due to low molecular weight. Consider benefits of breastfeeding and mother's need for treatment. |
■ FDA Black Box Warning
No FDA boxed warning.
| Serious Effects |
Severe renal impairment (eGFR <30 mL/min) not recommendedSevere hepatic impairment (Child-Pugh C)Hypersensitivity to nirmatrelvir, ritonavir, or any componentDrugs highly dependent on CYP3A for clearance where elevated levels are associated with serious events (e.g., alfuzosin, pethidine, propoxyphene, ranolazine, amiodarone, dronedarone, flecainide, propafenone, quinidine, colchicine, lomitapide, lovastatin, simvastatin, sildenafil for PAH, triazolam, midazolam oral, ergot derivatives)
| Precautions | Risk of serious adverse reactions due to drug interactions leading to potentially toxic levels of concomitant medications, Hepatotoxicity: hepatic transaminase elevations, hepatitis, and jaundice reported, HIV-1 protease inhibitor resistance: ritonavir may select for resistant HIV-1 variants in untreated patients, Allergic reactions: hypersensitivity including urticaria, angioedema, and anaphylaxis |
| Food/Dietary | No specific dietary restrictions. Can be taken with or without food. Avoid grapefruit products (grapefruit, grapefruit juice, Seville oranges) during treatment as they may increase drug levels of ritonavir. |
Loading safety data…
| No specific dose adjustment based solely on age. Dosing follows renal function as per renal adjustment criteria. Monitor for drug interactions and adverse effects due to potential polypharmacy and reduced renal function. |
| Lactation Rating |
| L3 (Moderately Safe) |
| Teratogenic Risk | Paxlovid (nirmatrelvir/ritonavir) is not recommended for use during pregnancy unless the potential benefit outweighs the potential risk. There are no adequate and well-controlled studies in pregnant women. Animal studies with nirmatrelvir showed no evidence of fetal harm at exposures up to 6 times the human exposure at the recommended dose; ritonavir is known to cross the placenta and has been associated with a slight increase in preterm delivery and lower birth weight in some studies. Fetal risks are not fully characterized; avoid in first trimester unless necessary. |
| Fetal Monitoring | Monitor for hepatotoxicity (LFTs), renal function, and potential drug-drug interactions. In pregnancy, monitor fetal growth and well-being via serial ultrasounds if indicated. Observe for symptoms of pancreatitis and hypersensitivity reactions. No specific fetal monitoring is mandated, but standard prenatal care should continue. |
| Fertility Effects | No human data on effect on fertility. Animal studies with nirmatrelvir showed no effect on male or female fertility at exposures up to 6 times the human exposure. Ritonavir has been associated with menstrual irregularities and potential impairment of fertility in some studies, but data are limited. |
| Clinical Pearls | Initiate PAXLOVID as soon as possible after COVID-19 diagnosis, within 5 days of symptom onset. Contraindicated in severe renal (eGFR <30 mL/min) or severe hepatic impairment (Child-Pugh C). Dose adjustment for moderate renal impairment (eGFR 30-59 mL/min): reduce nirmatrelvir to 150 mg. Monitor for hepatotoxicity and HIV-1 protease inhibitor resistance in uncontrolled HIV. Avoid in patients with suspected or confirmed COVID-19 who require hospitalization. Significant drug interactions with CYP3A4 substrates (e.g., rivaroxaban, midazolam, amiodarone) – consider alternative or hold. Use with caution in patients with risk factors for QT prolongation. |
| Patient Advice | Take PAXLOVID exactly as prescribed: two nirmatrelvir tablets (150 mg each) and one ritonavir tablet (100 mg) together twice daily for 5 days. · Start this medication as soon as possible after your COVID-19 diagnosis, ideally within 5 days of symptoms. · Do not skip doses or stop early even if you feel better; complete the full course. · If you miss a dose within 8 hours, take it as soon as you remember. If more than 8 hours, skip it and resume at regular time. · Store at room temperature away from moisture and heat. · This medication may interact with many other drugs, including certain statins, blood thinners, and heart medications. Provide a full list of your medications to your healthcare provider. · Contact your doctor immediately if you experience signs of liver problems: yellowing of skin or eyes, dark urine, pale stools, nausea, vomiting, or right upper belly pain. · PAXLOVID does not prevent transmission of COVID-19 – continue to practice infection control measures. · If you have kidney disease, your dose may need adjustment – inform your doctor about your kidney function. |