The aim of the study is to investigate if the cocktail of tramadol, omeprazole, losartan and caffeine can be used to simultaneously determine the activity of CYP2D6, CYP2C19, CYP2C9 and CYP1A2. Furthermore, will the natural occurring 4-beta-hydroxy-cholesterol in the blood be measured as a metric for CYP3A4/5.