"Muscle spasm" covers two different problems with different causes, and cannabis performs very differently against them.
Spasticity is continuous involuntary muscle tightness caused by damage to the nerve pathways controlling muscle tone. It appears in multiple sclerosis, spinal cord injury, cerebral palsy and after stroke. The muscle is behaving normally, the signalling telling it what to do is not.
Cramp and ordinary spasm is transient painful contraction, usually from overuse, dehydration, electrolyte disturbance or muscle strain. The problem is local.
Cannabinoid evidence exists for the first and barely exists for the second.
Spasticity: Where the Evidence Is
Multiple sclerosis spasticity is the best-documented cannabinoid indication in neurology. Nabiximols, an oromucosal spray with roughly equal THC and CBD, is approved in many countries for moderate to severe MS spasticity that has not responded adequately to other treatment.
The 2017 National Academies review found substantial evidence that oral cannabinoids improve patient-reported MS spasticity. The same review noted that clinician-measured spasticity scales frequently failed to show a matching change, which is a real caveat. Scales like the Ashworth measure resistance to passive limb movement in a clinic and may not capture spasm frequency or painful night-time cramping.
Spinal cord injury spasticity has less evidence than MS but is a plausible extension, since the mechanism is similar. Small studies have reported reduced spasticity and improved comfort. Existing treatment includes baclofen, tizanidine and botulinum toxin injections, all with meaningful limitations, which is why alternatives get tried.
Why Cannabinoids Affect Muscle Tone
CB1 receptors are present in the spinal cord and in motor control pathways of the brain, including the basal ganglia and cerebellum. Activating them dampens excitatory signalling in reflex arcs that become overactive when descending inhibitory control is damaged.
That is a plausible mechanism for spasticity specifically. It is much less relevant to a calf cramp after a run, where the problem is not central signalling at all.
Ordinary Cramps and Strains
There is essentially no evidence supporting cannabis for exercise cramps, muscle strains or general muscle tightness. Cannabis is not a peripheral muscle relaxant in the way methocarbamol or cyclobenzaprine are marketed to be.
What cannabis can do is change pain perception, so a strain may hurt less while healing at the same rate. That has a specific risk: reduced pain feedback during an injury can allow you to use the muscle in ways that worsen it.
For cramp specifically, the effective interventions are unglamorous: hydration, electrolyte correction, stretching, and addressing the underlying cause. Cannabis addresses none of them.
Topicals
Topical cannabinoid products are widely sold for muscle soreness. Cannabinoid receptors do exist in peripheral tissue, and it is genuinely unclear how much crosses skin to reach muscle at useful concentrations.
Two honest notes. Most of these products also contain menthol, camphor or capsaicin, which produce definite sensory effects independent of any cannabinoid. And the massage involved in applying them has its own benefit. Neither is a reason not to use them, and both should temper conclusions about what the cannabinoid contributed.
Practical Guidance
Bottom Line
Cannabinoids have substantial evidence for patient-reported spasticity in multiple sclerosis and an approved medicine for it, with plausible extension to spinal cord injury. For everyday cramps and strains there is no meaningful evidence, and reduced pain feedback during injury is a real downside. Establish which problem you have before deciding.
Disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult with a qualified healthcare professional before making any health-related decisions.