This MiniReview is a personal recollection of selected research topics, which the author in collaboration with colleagues has studied, aiming to improve the predictability of drug therapy. In early studies, we found bi- and trivalent cations to reduce the absorption of various tetracyclines and fluoroquinolones. Certain antacids elevated the bioavailability of some non-steroidal anti-inflammatory drugs and sulphonylureas. Various brands of phenytoin tablets revealed great differences in their bioavailability, causing clinical consequences. Numerous factors affecting the antidotal effect of activated charcoal were also studied, with charcoal compared to other gastrointestinal decontamination methods, including ipecac and gastric lavage. Effect of age and diseases on the pharmacokinetics of drugs was a research topic. Acute sotalol intoxications revealed its QT-prolonging properties, and even small mixed overdoses of moclobemide with serotonergic drugs proved fatal. Itraconazole and other potent inhibitors of CYP3A4 could drastically increase exposure to drugs like midazolam, triazolam, buspirone, lovastatin, simvastatin and oxycodone, whereas rifampicin greatly reduced their plasma concentrations. A change from potent inhibition to induction caused a 400-fold change in the exposure to oral midazolam. CYP2C8 was revealed to be crucial in the metabolism and interactions of several drugs. Many interactions affecting statins are CYP3A4-mediated, but transporters are important in certain interactions. Tizanidine is very susceptible to CYP1A2 inhibition. Fruit juices such as grapefruit juice can raise or lower exposure to different drugs. Both drug interactions and pharmacogenetics can modify the activity of cell membrane transporters and cause variability in the pharmacokinetics of and response to their substrate drugs.