The control of onchocerciasis remains a priority in sub-Saharan Africa. Jean-Philippe Chippaux, Michel Boussinesq, Jacques Gardon, Nathalie Gardon-Wendel and Jean-Christophe Ernould here outline studies concerning possible severe adverse reactions to ivermectin used to control onchocerciasis in areas where loiasis is also endemic, and discuss precautions that may be advisable during the implementation of such programmes.