The inhibition of CYP1A2, CYP2D6 and CYP3A4 by levomepromazine, demonstrated in vitro in the present study, should also be observed in vivo (especially the CYP2D6 inhibition by levomepromazine), since the calculated K(i) values are below or close to the presumed concentration range for levomepromazine in the liver in vivo. Therefore pharmacokinetic interactions involving levomepromazine and CYP2D6, CYP1A2 or CYP3A4 substrates are likely to occur in patients during co-administration of the above-mentioned substrates/drugs.