The effect of botulinum toxin (BT) upon the human body has so far been measured by using clinical scales monitoring its overall therapeutic effect upon the disorders treated. Clinical scales, however, usually lack sensitivity, are rarely validated and are integrating a number of uncontrollable parameters. After validation of the methodology in a group of 10 controls, we investigated the BT-induced amplitude reduction of the maximal voluntary electromyographic activity (M-EMG amplitude reduction) of the sternocleidomastoid muscle in a group of 34 patients with cervical dystonia undergoing regular BT therapy with Botox (Allergan, Irvine, Calif., USA; n = 16) or Dysport (Ipsen, Maidenhead, UK; n = 18). With Botox doses of 20 mouse units the M-EMG amplitude reduction was 80% (SD = 3.9%, n = 4), with 40 it was 84% (SD = 10.8%, n = 4), with 60 it was 85% (SD = 2.6%, n = 2) and with 80 it was 91% (SD = 5.8%, n = 6). With Dysport doses of 100 mouse units the M-EMG amplitude reduction was 70% (SD = 7.6%, n = 4), with 200 it was 85% (SD = 10.4%, n = 5), with 300 it was 83% (SD = 9.2%, n = 3), with 400 it was 78% (SD = 6.7%, n = 3) and with 500 it was 91% (SD = 5.8%, n = 5). The methodology presented can measure M-EMG amplitude reductions with a precision of about 10%. Dose-efficacy relationships can be used for dose optimisation, evaluation of BT therapy failure and comparison of different preparations and types of BT.