Botulinum toxin is primarily indicated on the upper third of face. The treatment of the forehead wrinkles has a limited number of side effects, but can lead to brow ptosis. Brow ptosis appears in many studies with a frequency of around 1-5%. This is caused, often, by using incorrect injection sites, too high dosages, and by an inappropriate selection of patients. In our experience, this side effect has occurred in less than 1% of cases. This paper will emphasize the technical methods we use in order to reduce to a minimum this unpleasant side effect. Understanding the side effects of botulinum toxin on forehead requires a thorough understanding of muscular anatomy. We emphasize the importance of the correct dilution, the use of adrenalin and a 30 U syringe, particularly for patients who have skin abundance (dermatocalasys) of forehead. Patients with dermatocalasys should usually be excluded, but if treated, it should be done in two different sessions, without overdosing and respecting the symmetry. Brow ptosis, the only real side effect of forehead treatment with botulinum toxin A, can be reduced to a minimum by using simple rules.