Pfizer antiviral pills for Covid may be risky when taken with other medications, experts warn

FDA says drug combination is not recommended for people with severe kidney and liver disease

Pfizer’s new antiviral pills against Covid-19 may not be safe for everyone, experts have cautioned, adding that the new drug could be life-threatening when taken with other medications.

The US Food and Drug Administration (FDA) recently authorised Pfizer’s pill, Paxlovid, for emergency use in those with mild or moderate Covid-19 who are more likely to become seriously ill, including older people and those with underlying health conditions such as heart disease, cancer, or diabetes.

Paxlovid’s treatment is a combination of two pills — the antiviral nirmatrelvir and one tablet of ritonavir — taken over five days, which Pfizer said showed near 90 per cent efficacy in preventing hospitalisations and deaths in high-risk patients.

Merck’s antiviral pill Molnupiravir, reportedly less efficacious than Pfizer’s, is restricted for use only in adults when other authorised treatments such as monoclonal antibodies are inaccessible or are not “clinically appropriate.”

Both Pfizer’s and Merck’s antiviral pills aren’t authorised for people already hospitalised with Covid-19, and are to be taken as soon as a person feels symptoms.

Molnupiravir is not authorised for children as it may interfere with bone growth and is also not recommended for pregnant women due to potential birth defect possibilities.

In a new fact sheet, the FDA has issued a list of medications that should not be taken along with the Paxlovid cocktail.

It says the drug combination is not recommended for people with severe kidney and liver disease as one of the drugs in the antiviral cocktail may cause severe consequences with widely-used medications such as statins and antidepressants.

Ritonavir — one of the drugs part of the Paxlovid cocktail — has been shown to suppress a liver enzyme called CYP3A, which breaks down many medications, including nirmatrelvir, helping retain the antiviral in the body at therapeutic levels for longer durations.

However, experts caution that this effect may boost other co-administered drugs to toxic levels. “Therefore, caution should be exercised when administering Paxlovid to patients with pre-existing liver diseases, liver enzyme abnormalities, or hepatitis,” the FDA noted.

Since one of the drugs in the combination, ritonavir, is also an HIV-1 enzyme inhibitor, experts caution Paxlovid use may lead to the virus developing resistance to this class of enzyme inhibitors in individuals with undiagnosed or uncontrolled HIV-1 infection.

They say that people who have been prescribed Pfizer’s or Merck’s new medications will require careful monitoring by doctors and pharmacists.

“Its effect on drug metabolism may result in drug interactions, and some drugs may be contraindicated. However, in light of the fact that Paxlovid has a short duration of treatment of five days, combined with a low dose of Ritonavir of 100 milligrams, we believe that healthcare professionals should find most DDIs to be generally manageable,” a Pfizer spokesperson told The Hill.

“Healthcare providers should consider the potential for drug interactions prior to and during Paxlovid therapy and review concomitant medications during Paxlovid therapy,” the spokesperson added.

But since the Paxlovid treatment regimen is brief, experts say the risk of adverse interactions with other medications could be manageable.

“Pharmacists are highly trained experts in medication safety and monitoring and are an excellent source of information and advice about interactions between medications and also supplements and herbal products,” Emily Zadvorny, a clinical pharmacist who is the executive director of the Colorado Pharmacists Society, told NBC News.

“They will help determine if a significant interaction exists and devise solutions to mitigate the interaction if possible,” Dr Zadvorny added.