Paxlovid contains two separate medications packaged together: nirmatrelvir and ritonavir. Each dose includes two pink nirmatrelvir tablets (150 mg each, totaling 300 mg) and one white ritonavir tablet (100 mg). You take all three tablets together, twice a day, for five days.
Only one of these two drugs actually fights the virus. The other is there purely to keep the first one working long enough to be effective. Understanding what each ingredient does, and what else is in the tablets, matters if you have allergies, kidney problems, or take other medications.
Nirmatrelvir: The Antiviral Component
Nirmatrelvir is the ingredient that targets SARS-CoV-2 directly. The virus relies on a specific enzyme (called its main protease) to chop up long chains of protein into the smaller functional pieces it needs to replicate. Nirmatrelvir locks onto that enzyme and permanently disables it by forming a chemical bond with it. Without a working protease, the virus can’t assemble new copies of itself, and the infection stalls.
Each nirmatrelvir tablet is 150 mg, and a standard dose requires two of them. The tablets are pink, colored by iron oxide in the film coating. Other inactive ingredients in the nirmatrelvir tablets include lactose monohydrate, microcrystalline cellulose, croscarmellose sodium, colloidal silicon dioxide, and sodium stearyl fumarate, with a film coating that also contains titanium dioxide and polyethylene glycol.
Ritonavir: The Booster
Ritonavir has no activity against the COVID-19 virus. It’s included for one reason: your liver breaks down nirmatrelvir too quickly on its own. Ritonavir blocks the liver enzyme responsible for that breakdown, which keeps nirmatrelvir circulating in your blood at high enough levels to suppress the virus. This “boosting” strategy has been used for decades in HIV treatment, where ritonavir was originally developed as an HIV protease inhibitor.
The ritonavir tablet is 100 mg and is white. Its inactive ingredients include anhydrous dibasic calcium phosphate, colloidal silicon dioxide, copovidone, sodium stearyl fumarate, and sorbitan monolaurate. The film coating may contain hydroxypropyl cellulose, hypromellose, polyethylene glycol, polysorbate 80, talc, and titanium dioxide.
Why the Ritonavir Component Causes Drug Interactions
The same liver enzyme that ritonavir blocks (CYP3A) is also responsible for processing a long list of common medications. When ritonavir suppresses that enzyme to boost nirmatrelvir, it simultaneously causes other drugs to build up to dangerously high levels in your body. This is the main safety concern with Paxlovid.
Medications that cannot be taken with Paxlovid include certain heart rhythm drugs (amiodarone, flecainide, propafenone), cholesterol-lowering statins (lovastatin and simvastatin, though these can be temporarily paused), certain sedatives (triazolam, oral midazolam), some migraine medications, and the anti-gout drug colchicine in people with kidney or liver problems. Ergot derivatives used for migraines are also contraindicated.
The interaction works in the other direction too. Drugs that rev up CYP3A activity, called strong inducers, can break down nirmatrelvir and ritonavir so fast that Paxlovid stops working. These include the seizure medications carbamazepine, phenytoin, and phenobarbital, the antibiotic rifampin, and the herbal supplement St. John’s Wort. Paxlovid can’t be started immediately after stopping any of these because their enzyme-boosting effects linger.
Dose Adjustments for Kidney Function
People with moderate kidney impairment take a reduced dose: one nirmatrelvir tablet (150 mg) instead of two, still paired with one ritonavir tablet (100 mg), twice daily for five days. The ritonavir dose stays the same because the adjustment is only about how much antiviral your kidneys can handle.
If you pick up Paxlovid at a pharmacy with a reduced dose, the pharmacist will have physically removed the extra nirmatrelvir tablets from each blister card and covered the empty spaces with stickers that include updated dosing instructions. This prevents confusion since the standard packaging is designed around the full dose.
What the Packaging Looks Like
Paxlovid comes in a carton containing five blister cards, one for each day of treatment. Each card has a morning row and an evening row. In the standard dose, each row holds two pink nirmatrelvir tablets and one white ritonavir tablet. You pop out all three from one row, take them together (with or without food), and repeat 12 hours later with the other row.
FDA Approval and Shelf Life
Paxlovid received full FDA approval in May 2023 for adults at high risk of severe COVID-19. For adolescents aged 12 and older (weighing at least 40 kg), it remains available under emergency use authorization. As of March 2024, only commercially manufactured Paxlovid labeled under the approved application can be dispensed. Earlier EUA-labeled stock is no longer authorized regardless of its expiration date.
The shelf life has been extended several times since the drug’s initial authorization. It currently stands at 24 months from the earliest manufacture date between the two components, provided the product has been stored according to labeled conditions. If you have Paxlovid at home, check the manufacturer’s website or the FDA’s expiration dating tables to confirm whether your specific lot is still within date.