We examined the effect of over-the-counter (OTC) conversion of prescription drugs on utilization at the drug class level using monthly drug utilization data from the US for the period 1999-2010 for 9 drug classes: antihistamines, benign prostatic hyperplasia medication, cholesterol control drugs (statins), analgesics (triptans), contraception medications (emergency contraception), antiulcerants (proton pump inhibitors) non-sedating antihistamines, weight-loss remedies and erectile dysfunction remedies. We performed interrupted time series analysis to detect a break in the trend of drug utilization following OTC introduction. We found that the introduction of the first OTC drug increased drug utilization at the class level by an average of 30% or more. We concluded that OTC switches can be an important policy tool for improving public health in drug classes where a significant proportion of the population is untreated and where consumers can effectively manage treatment with limited physician supervision.