Plasma exchange has been proposed for treating diseases mediated by circulating immune complexes. Removal of drugs during plasma exchange is a complex function of pharmacokinetic characteristics and specifications of the plasma exchange procedure: the time of plasma exchange, the filtered volume of plasma, the number of procedure. The effect of plasma exchange on the kinetics of drugs can be evaluated by different parameters: amount eliminated, extracorporeal clearance, fractional drug removal, extracorporeal elimination rate constant. Knowledge of the impact of plasma on the elimination of drugs is essential to the design of the dosage regimen in patients treated with plasma exchange. The fraction of the estimated amount in body removed by plasma exchange is the best parameter to evaluate the effectiveness of the exchange procedure, but it is necessary to know the proportion of body pool of drug at the start of the plasma exchange. Drug recovery by each exchange may account for 0.5%-30% of the dose. Controlled studies are needed to quantify the effects of plasma exchange on drug therapy. Dosage adjustment is sometimes required.