Methylxanthines, purine alkaloids found in plants, are found in beverages (coffee, tea, cocoa) and foods (chocolate and other cocoa-containing foods) commonly consumed worldwide. Members of this family include caffeine, theophylline and theobromine. Methylxanthines have a variety of pharmacological effects, and caffeine and theophylline are used as pharmaceuticals. Methylxanthines are metabolized in the liver predominantly by the enzyme CYP1A2. Their co-administration with CYP1A2 inhibitors may lead to pharmacokinetic interactions. Little is known about the possible drug interactions between caffeine and substrates of other CYP450 enzymes. In our study, methylxanthine fractions inhibited CYP3A4 in a concentration-dependent manner. Concomitant consumption of green tea with CYP3A4 substrates could increase the possibility of interactions, and this requires further clarification. The inhibition of CYP3A4 is not only due to the presence of catechin derivatives but methylxanthines may also contribute to this effect.