Paxlovid is an antiviral pill that blocks a key enzyme the COVID-19 virus needs to copy itself, reducing the risk of hospitalization by roughly 68% and death by about 88% in high-risk patients. It’s a five-day course of two medications taken together: nirmatrelvir, which does the actual antiviral work, and ritonavir, which keeps nirmatrelvir in your bloodstream long enough to be effective.
How Paxlovid Stops the Virus
When SARS-CoV-2 infects a cell, it produces long chains of proteins that need to be cut into smaller functional pieces before the virus can replicate. The virus relies on a specific enzyme, called the main protease, to make those cuts at 11 different sites along the protein chain. Without those cuts, the virus can’t assemble the machinery it needs to make copies of itself.
Nirmatrelvir, the active antiviral ingredient in Paxlovid, latches onto this enzyme and temporarily disables it. It forms a bond with the enzyme’s active site, physically blocking it from doing its job. Because this bond is reversible, nirmatrelvir doesn’t permanently destroy the enzyme, but it holds on long enough to stall viral replication during the critical early days of infection when the virus is multiplying fastest.
The second pill in the pack, ritonavir, has no direct antiviral activity against COVID-19. Its entire purpose is pharmacological: it blocks a liver enzyme that would otherwise break down nirmatrelvir too quickly. Without ritonavir, nirmatrelvir’s levels in your blood would drop below the concentration needed to suppress the virus between doses. Ritonavir is a potent and irreversible blocker of this liver enzyme, reaching maximum effect within about 48 hours. After you stop taking it, the inhibition takes roughly three days to fully wear off as your body generates fresh enzymes.
Who Can Take It
Paxlovid is approved for adults and for adolescents aged 12 and older who weigh at least 88 pounds (40 kg). It’s intended for people who test positive for COVID-19 and are at higher risk of progressing to severe illness, hospitalization, or death. There’s no single checklist of qualifying conditions. Instead, prescribers evaluate your individual risk based on factors the CDC associates with worse outcomes: older age, obesity, diabetes, heart disease, chronic lung conditions, weakened immune systems, and several others.
The treatment window is narrow. You need to start Paxlovid within five days of your first symptoms. The earlier you begin, the more effectively it curbs viral replication before the infection peaks. This means getting tested quickly and contacting a healthcare provider as soon as symptoms appear if you think you might qualify.
What the Treatment Looks Like
The standard course is two nirmatrelvir tablets (300 mg total) plus one ritonavir tablet (100 mg), taken together twice a day for five days. You take them with or without food. For people with moderate kidney impairment, the nirmatrelvir dose is cut in half to one tablet per dose. People with severe kidney impairment follow a different schedule: a higher loading dose on day one, then a reduced once-daily dose for the remaining four days.
Because ritonavir interferes with the same liver enzyme that processes many common medications, Paxlovid has a long list of drug interactions. Certain cholesterol-lowering drugs, blood thinners, heart rhythm medications, and sedatives can reach dangerously high or ineffectively low levels when taken alongside ritonavir. If you take regular medications, your prescriber will need to review them before writing the prescription. In some cases, you may need to pause a medication for the five-day course and a few days afterward while ritonavir’s effects wear off.
How Well It Works
The landmark clinical trial enrolled unvaccinated, high-risk adults who started treatment within five days of symptoms. Among those who received Paxlovid, 5 were hospitalized and none died. In the placebo group, 44 were hospitalized and 9 died. A meta-analysis pooling data from multiple studies calculated an 88% reduction in mortality and a 68% reduction in hospitalization. These are striking numbers, though they reflect the population most likely to benefit: people with risk factors for severe disease who were treated early.
For people who are vaccinated, younger, or otherwise at lower risk, the absolute benefit is smaller because the baseline risk of hospitalization is already low. Paxlovid still reduces viral replication in these groups, but the difference between treatment and no treatment is less dramatic when severe outcomes are already uncommon.
Side Effects
The most distinctive side effect is a bitter, metallic, or otherwise unpleasant taste that many people notice throughout the five-day course. In pharmacovigilance data, about 17.5% of Paxlovid users reported this taste disturbance, and the effect appears specific to nirmatrelvir rather than being a symptom of COVID-19 itself or a side effect of ritonavir alone. Women reported it more frequently than men. The taste typically resolves after you finish the course.
Other commonly reported side effects include diarrhea, muscle aches, and elevated blood pressure, though these overlap considerably with COVID-19 symptoms themselves, making it hard to pin down what’s caused by the drug versus the infection.
COVID Rebound After Treatment
Some people test negative or feel better after finishing Paxlovid, only to see symptoms return or tests turn positive again a few days later. This “rebound” phenomenon received significant media attention, but the data tell a more nuanced story. In a CDC analysis of the original clinical trial data, viral RNA rebound occurred in about 6% to 8% of people who took Paxlovid, compared to roughly 6% to 7% of people who took a placebo. The difference was not statistically significant once the analysis was restricted to people who had responded to treatment by day five.
In other words, rebound happens after COVID-19 infection regardless of whether Paxlovid is involved. The drug doesn’t appear to cause rebound at a meaningfully higher rate than the natural course of the illness. When researchers looked specifically at rebound that reached levels associated with actual infectiousness (rather than just detectable viral fragments), rates were similar between treated and untreated groups. Rebound episodes are generally mild and resolve on their own within a few days.