In discussing undesirable therapy effects by thyrostatics one must issue from the fact that the possible side effects have a different genesis and appear in different frequency. While pharmacodynamically caused therapy effects are generally to be avoided, when considering the pathophysiology of the regulation of the thyroid gland, this is not the case as to allergic side effects and only to a limited extent with regard to toxic side effects. Allergic side effects abruptly and suddenly appear and do not show any dependence on dosage. But for the greater part these side effects are harmless. Severe side effects are nearly without exception of toxic genesis. The decrease of the initial dose and the early transition to a still effective maintenance dose reduce the possibility of toxic complications. In case that side effects lead to a discontinuation of the corresponding thyrostatic, a change to a thyrostatic of another chemical group is possible and necessary.