The antiviral medication Paxlovid interacts adversely with many medications meant for heart conditions and post-transplant patients, narrowing the pool of people who could benefit from the drug.

Paxlovid comprises two medications, one used as an enhancer for the other. That medication, ritonavir, can either boost or lower concentrations of other drugs in a person’s system, generating the risk that other drug toxicities will become dangerously too high. Those patients will require close monitoring while taking the medication.