Skeletal muscle relaxants and cannabinoids for pain

Muscle relaxants are short-term options for acute back pain, not long-term analgesics; avoid them in older adults, never pair tizanidine with strong CYP1A2 inhibitors, and treat cannabinoid efficacy claims for pain as unproven.

1. Acute back pain is the evidence-based niche. ACP strongly recommends nonpharmacologic care first for acute or subacute low back pain; if drugs are wanted, NSAIDs or skeletal muscle relaxants are the choices (moderate-quality evidence) ¹.
2. Carisoprodol is short-course and scheduled. It is Schedule IV, labeled for acute painful musculoskeletal conditions, and limited to two or three weeks because longer effectiveness is not established ².
3. Tizanidine is a CYP1A2 substrate. The central alpha-2 agonist is contraindicated with strong CYP1A2 inhibitors such as fluvoxamine and ciprofloxacin, which sharply raise levels and cause hypotension, drowsiness and psychomotor impairment ³.
4. Older adults should avoid muscle relaxants. Beers lists cyclobenzaprine, carisoprodol, methocarbamol and similar drugs to avoid: poorly tolerated, anticholinergic, sedating, raising fracture risk, and of questionable effect at tolerated doses ⁴.
5. Cannabinoid evidence is weak. A 36-trial systematic review found all studies at high or uncertain risk of bias, with low or very low-quality outcomes; the evidence neither supports nor refutes efficacy and safety for pain ⁵.
6. Know the two positive signals. Benefit appeared only for cannabis used under 7 days (2 trials, risk difference 0.33) and nabiximols used over 7 days (6 trials, risk difference 0.06), both very low quality ⁵.

Stopping tizanidine abruptly or co-prescribing it with ciprofloxacin, fluvoxamine or another alpha-2 agonist; taper it slowly and monitor blood pressure, especially with antihypertensives ³.

• Moderate or weak CYP1A2 inhibitors should also be avoided with tizanidine; they may cause hypotension, bradycardia or excessive drowsiness ³.
• Tizanidine needs aminotransferases at baseline and 1 month after maximum dose; stop it if liver injury occurs ³.
• Carisoprodol is contraindicated in acute intermittent porphyria and in carbamate (meprobamate) hypersensitivity ².
• In low back pain trials, 13–17% of carisoprodol patients reported sedation versus 6% with placebo ².
• Cannabis, nabiximols and THC caused more adverse events than control ⁵.
• No benefit was found for the other cannabinoid, cannabis or cannabis-based medicine comparisons; 81% of subgroup analyses were negative ⁵.
• Tizanidine starts at 2 mg every 6–8 hours as needed, up to 3 doses in 24 hours, rising by 2–4 mg per dose ³.
• For chronic low back pain, ACP recommends starting with nonpharmacologic therapy such as exercise, multidisciplinary rehabilitation or mindfulness ¹.

• Treating a Beers "avoid" listing for muscle relaxants as permission for short courses; the criterion is to avoid ⁴.
• Forgetting that common antibiotics such as fluoroquinolones inhibit CYP1A2 and so raise tizanidine exposure ³.
• Continuing carisoprodol for chronic back pain ².
• Quoting cannabinoids as proven analgesics ⁵.