["Clinically significant" new drug interactions].

The concomitant intake of drugs with the potential to cause drug interactions is frequent. In contrast, adverse effects due to drug interactions account for only a small fraction of all adverse effects. A reproducible evaluation of the clinical relevance of drug interactions is lacking. We now can accurately define the potential of a drug to cause interactions, primarily by comparative investigations within a drug class. Whether or not the selection of the drug based on this information is useful for the patient is unknown. Therefore, usually it is to be recommended to abandon therapeutically reasonable drug combinations with a risk for interactions only if equivalent therapeutic options are available. Several actual examples on interactions with selective serotonin re-uptake inhibitors, HMG-CoA reductase inhibitors, mibefradil, sildenafil, protease inhibitors and with grapefruit juice are discussed.