279 patients diagnosed with multiple sclerosis (MS) and experiencing muscle stiffness were randomly assigned to receive either oral capsules of delta-9-tetrahydrocannabinol (maximum dose per day: 25mg) or placebo capsules for 12 weeks. Patients were assessed on the proportion reporting relief (0-3 on a 11-point Category Rating Scale) in muscle stiffness, muscle spasm, body pain, and sleep quality at 4-week, 8-week, and 12-week post treatment, as well as the incidence of adverse events and other measurements of physical and psychological impact. Results demonstrated that delta-9-tetrahydrocannabinol was effective in relieving muscle stiffness, muscle spasm, and sleep quality 12 weeks post-treatment, without significant proportion of serious adverse events and psychological impact.