Serious safety issues relating to drugs are communicated to health-care professionals via Direct Health-Care Professional Communications (DHPCs). We explored which characteristics determined the impact of DHPCs issued in the Netherlands for ambulatory-care drugs (2001-2008). With multiple linear regression, we examined the impact on the relative change in new drug use post-DHPC of the following: time to DHPC, trend in use, degree of innovation, specialist drug, first/repeated DHPC, DHPC template, and type of safety issue. DHPCs have less impact on use of specialist drugs than nonspecialist drugs (P <0.05).The DHPCs' impact increased after availability of a template emphasizing the main problem (P <0.05), and for safety issues with a risk of death and/or disability (both P <0.05) (adjusted R-2 = 0.392). Risk communication can be effective, specifically in case of well-structured information, and very serious safety issues. Effectiveness may improve by tailoring DHPCs and adding other communication channels, for example for drugs that are increasingly being used.