Background: In hemodialyzed patients, physicians have to (1) adjust drug dosage for a creatinine clearance lower than 10–15 ml/min and (2) know whether or not the drug will be removed by the dialysis session to decide whether it may be administered before or after the session on dialysis days. However, of several indices being used to evaluate drug removal by dialysis none is appropriate and we suggest a novel index named F<sub>HD</sub>, which reflects the role of hemodialysis clearance of a drug in its overall clearance during the session. Methods: Pharmacokinetic simulations were performed to test the influence of dialysis on the pharmacokinetics of some drugs, whether F<sub>HD</sub> was considered or not, to determine when to administer the drug. F<sub>HD</sub> was then calculated for several drugs and its value compared with other indices. Five hemodialysis patients from our department for whom the time of drug administration was determined according to F<sub>HD</sub> were included in a small study and their drugs’ trough concentrations were monitored. Results: F<sub>HD</sub> emphasized that considering hemodialysis clearance alone may lead to false interpretations of the potential dialyzability of some drugs. In our patients, who received their treatment according to the ‘F<sub>HD</sub> rule’, monitoring of trough levels gave satisfactory results. Conclusion: The use of the ‘F<sub>HD</sub> rule’ should be tested on a long-term administration basis to confirm our conclusion. F<sub>HD </sub>could be the index of choice to determine when to administer a drug, before or after the session, in hemodialysis patients.