Ergotamine and analgesic misuse are now recognized as causes of chronic daily headache and the condition responds well to drug withdrawal with reduced headache frequency. In this study, we have investigated whether medication misuse is associated with an alteration in membrane transduction which is sensitive to drug withdrawal. This was carried out by assay of the thrombin-stimulated generation of inositol phosphates in platelets from 12 migraine patients with chronic daily headache and analgesic misuse, 7 migraine patients with chronic daily headache and ergotamine misuse and 7 control subjects. After drug withdrawal, a significant decrease in headache frequency was seen at one month in both patient groups. Withdrawal of analgesics produced a significant decrease in thrombin-stimulated inositol phosphate production at one month; this was further decreased a month later with a reduction in Bmax of 60% and no significant change in KD. A similar pattern was obtained in ergot misuse patients, with the KD value decreasing by 56% one month after drug withdrawal. These results provide evidence of an adaptation in transduction with misuse of analgesics and ergotamine which correlates with headache frequency.