Summary:Purpose: Topiramate (TPM) is an antiepileptic agent, first licensed in the United Kingdom in 1994, that is used in the treatment of patients with refractory seizure disorders. TPM is a monosaccharide D-fructose derivate, with sulfamate function, and so far, few adverse side effects have been reported.Methods: We describe three patients with epilepsy who were treated with TPM and developed hypohidrosis, heat and exercise intolerance, as well as fever. The sudomotor function was assessed after peripheral stimulation with pilocarpine iontophoresis.Results: Sweat response was reduced in all three patients. Signs and symptoms ceased after drug suppression.Conclusions: This side effect associated with TPM, which has not been described previously, can be clinically significant during heat stress and exercise challenge.