Study design: Open label study to determine drug dose for a randomized double-blind placebocontrolled parallel study. Objectives: To assess the efficacy and side effects of oral D 9 -tetrahydrocannabinol (THC) and rectal THC-hemisuccinate (THC-HS) in SCI patients. Setting: REHAB Basel, Switzerland. Method: Twenty-five patients with SCI were included in this three-phase study with individual dose adjustment, each consisting of 6 weeks. Twenty-two participants received oral THC open label starting with a single dose of 10 mg (Phase 1, completed by 15 patients). Eight subjects received rectal THC-HS (Phase 2, completed by seven patients). In Phase 3, six patients were treated with oral THC and seven with placebo. Major outcome parameters were the spasticity sum score (SSS) using the Modified Ashworth Scale (MAS) and self-ratings of spasticity. Results: Mean daily doses were 31 mg with THC and 43 mg with THC-HS. Mean SSS for THC decreased significantly from 16.72 (77.60) at baseline to 8.92 (77.14) on day 43. Similar improvement was seen with THC-HS. We observed a significant improvement of SSS with active drug (P ¼ 0.001) in the seven subjects who received oral THC in Phase 1 and placebo in Phase 3. Major reasons for drop out were increase of pain and psychological side effects. Conclusion: THC is an effective and safe drug in the treatment of spasticity. At least 15-20 mg per day were needed to achieve a therapeutic effect.