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.