This study examines the way side effects information is presented in patient information leaflets about drugs. In a field experiment, we tested the effects of two attempts to improve a side effects paragraph in a leaflet about a nonsteroid anti-inflammatory drug. First, a short introductory passage on the nature of side effects was added. Second and more importantly, we changed the frequency descriptors (FDs) for the side effects. A preliminary study had shown that the frequencies associated with common Dutch FDs are much higher than the writers of patient information leaflets and package inserts mean to convey. In our experiment we replaced the original FDs by lower-assessed FDs. For instance, soms (sometimes) was replaced by zelden (seldomly). Replacing FDs led to lower recall for the side effects mentioned in the leaflet. It also decreased the number of side effects experienced. Contrary to our expectations, lower FDs did not significantly increase the confidence in the safety and effectiveness of the drug, nor did they increase therapy compliance; incompliance was extremely rare in our sample of patients. Adding an introductory passage on the nature of side effects lowered FD interpretations. It did not significantly affect any of the other dependent variables.