Rising pharmaceutical expenditures have led to the use of cost-sharing measures. The authors undertook a systematic review of the effects of cost sharing on vulnerable populations (the poor and those with chronic illnesses). Virtually every article reviewed supports the view that cost sharing decreases the use of prescription drugs in these populations. Copayments or a cap on the monthly number of subsidized prescriptions lower drug costs for the payer, but any savings may be offset by increases in other health care areas. Cost sharing also leads to patients foregoing essential medications and to a decline in health care status.