• Muscle stiffness in myotonic dystrophy (myotonia): Limited. In a 2020 German case series, 6 patients with treatment-resistant myotonia given a THC/CBD oil all reported less stiffness, and hand-opening time improved in 5 (Montagnese 2020). No controlled trial has been done.
• Muscle pain: Limited. Patients with myotonic dystrophy commonly report using cannabis for pain, but evidence is anecdotal (Myotonic Dystrophy Foundation). See Chronic pain.
• Slowing muscle loss (Duchenne and others): Not established. CBD and related compounds reduced inflammation and muscle damage in mice; there are no human studies (Iannotti 2019).

Compared with standard treatment: Corticosteroids (deflazacort, prednisone), vamorolone, and newer gene-targeted therapies for Duchenne; mexiletine for myotonia; physical therapy and heart and lung care for all types.

Good to know: Many forms of muscular dystrophy affect the heart and breathing. THC can speed the heart rate, so anyone with heart rhythm problems should talk with their cardiologist first, and inhaled products should be avoided when breathing muscles are weak. CBD can raise blood levels of some medicines, including deflazacort. Constipation was common in the case series.

What to ask for: No product has been tested in a controlled trial. For stiffness or pain, a low dose of an oral balanced THC/CBD oil, the type used in the case series, started under the neuromuscular specialist’s guidance.