The problem is the drug is inhibiting the CYP3A enzyme in the liver. Thus, drugs dependent on this enzyme for metabolism will not be excreted properly. For instance simvastatin uses this enzyme. So if you took 40mg of simvastatin day 1 on day 2 most of it will still be your blood so it is now like 80mg and Day 3 120mg Day 4 160mg. Now that example is a 100% blockage. It will not be that strong but it will cause drugs to get to toxic levels. The easy answer is you hold the other drugs while you take the new one. Once complete you can restart old medications. A significant amount will still be present by metabolism being significantly slowed down.

What Reveille said. But there may be some drugs that cannot be safely paused like antiarrhythmics and such. With those you can decrease/increase dosage or use alternatives that don't interact while on Paxlovid.