Patients with COVID‐19 are sometimes already being treated for one or more other chronic conditions, especially if they are elderly. Introducing a treatment against COVID‐19, either on an outpatient basis or during hospitalization for more severe cases, raises the question of potential drug–drug interactions. Here, we analyzed the potential or proven risk of the co‐administration of drugs used for the most common chronic diseases and those currently offered as treatment or undergoing therapeutic trials for COVID‐19. Practical recommendations are offered, where possible.

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The pharmacokinetic interactions of ofloxacin (2 X 200 mg) and theophylline (3 X 200 mg) were investigated in 12 healthy volunteers over a period of two weeks. In the first week, theophylline was given over five days to reach a steady state. In the second week, the combination of theophylline and ofloxacin was applied. Cmax, tmax, AUC0-8, the serum elimination constant and serum half-life of theophylline were not changed when theophylline was given alone or in combination with ofloxacin. The kinetic parameters of ofloxacin were in accordance with data from the literature.

There have been reports of an interaction when theophylline and macrolides are given together, and also when carbamazepine is given with macrolides. We compared the kinetics of theophylline and carbamazepine, given alone and then in combination with roxithromycin. Roxithromycin had little effect on the pharmacokinetics of theophylline and none on carbamazepine, and roxithromycin can be given with either of the drugs without any need to alter the dose.

The macrolide antibiotics are metabolized by cytochrome P-450 enzymes in the liver and interactions with similarly metabolized compounds have been described. Simultaneous treatment with erythromycin and warfarin is known to decrease warfarin clearance and prolong prothrombin time. Roxithromycin (RU 28965), a new erythromycin derivative with improved pharmacokinetic properties, might then, because of structure similarity, be expected to interact with warfarin. In 21 healthy volunteers, the effect of orally administered roxithromycin (150 mg b.i.d.) on warfarin steady-state kinetics, and the effects of warfarin on roxithromycin kinetics, were investigated in a double-blind, randomized study versus placebo. Since the warfarin enantiomers, R- and S-warfarin have both different potency and different metabolism, the ratio between the enantiomers with and without roxithromycin, was also determined. In this study, mean AUC for warfarin increased slightly from day 14 of warfarin treatment to day 28, but no difference was found between the roxithromycin group and the placebo group, and no change appeared in the ratio between the warfarin enantiomers. A moderate increase in dosage was needed to maintain hypocoagulability during warfarin medication, but there was no difference between the roxithromycin group and the placebo groups, respectively. In addition, roxithromycin kinetics appeared to be unaffected by warfarin treatment.