Covid warning as Pfizer pill has risk ‘of serious side effects’ when combined with tablet | Science | News

According to reports by US-based NBC News, “when Paxlovid is combined with other drugs that are also metabolised by the CYP3A enzyme, the main concern is that ritonavir may increase the co-administered drugs to toxic levels”.

Paxlovid was approved on 22 December to treat severe to moderate Covid in people over the age of 12.

It can be purchased with a prescription and patients must take it within five days of the onset of their symptoms.

Paxlovid is based on a combination of two separate pills.

One is the antiviral nirmatrelvir and the other is a ritonavir pill that has long been used as a stimulant in HIV treatment.

Ritonavir suppresses a liver enzyme called CYP3A, which metabolises many drugs, including nirmatrelvir.

When Paxlovid is used on a Covid patient, ritonavir helps slow down the breakdown of the active antiviral drug in the body.

It also helps it remain at a therapeutic level for longer, so the patient feels the effect of the drug and avoids serious illness. 

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However, experts say that the treatment is a ‘breakthrough’ and that drug-drug interactions are ‘manageable’.

They also argue that Paxlovid is only authorised for a maximum of five days and that patients should inform their doctor about other medications they are taking.

Additional reporting by Maria Ortega.

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