Numbers of patients who use easily available herbal medicines are growing rapidly. The drugs are often regarded to be a safe alternative when conventional therapies are not effective. Testing drug pharmacokinetics in vivo is of special importance. It is to be remembered that an interaction predicted theoretically and confirmed in vitro does not have to implicate a clinical importance. In the paper several important herb-drug interactions occurring in the pharmacokinetic phase are reported. Interactions occurring in the biotransformation phase are the most common and of particular importance, especially if cytochrome P450 (CYP) isozymes are involved. Determining changes in pharmacokinetics enables modifying doses of interfering drugs.