Short answer
Evidence that cannabis relieves nerve pain over years is thin. Most trials of cannabis for neuropathic pain run 12 weeks or less. Long-term use carries risks that grow with duration: tolerance, dependence, withdrawal, memory and attention effects, respiratory harm from smoking, and cannabinoid hyperemesis syndrome.
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What the clinical trials show
A 2018 Cochrane review pooled 16 randomized trials of cannabis-based medicines for chronic neuropathic pain in adults. Participants totaled about 1,750. The review found moderate-quality evidence that more people reached at least a 30% drop in pain scores than with placebo, and low-quality evidence for a 50% drop. Most trials lasted 2 to 15 weeks. The review authors wrote that longer trials were needed.
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Nabiximols, a THC and CBD mouth spray, and smoked cannabis were the products tested most. Nabiximols is not sold in the United States.
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What long-term data exist
There is no large randomized trial of cannabis for neuropathic pain that ran past one year. Long-term information comes from observational studies, patient registries, and studies of people who use cannabis for other reasons. Those designs cannot separate a drug effect from the traits of people who choose the drug.
Risks that grow with duration
Tolerance
Repeated THC exposure reduces response at CB1 receptors. Patients report that the same dose stops working. Dose escalation follows, which raises side effect risk.
Dependence and withdrawal
The National Institute on Drug Abuse reports that about 3 in 10 people who use cannabis develop cannabis use disorder. Risk is higher for people who start in their teens. DSM-5 lists cannabis withdrawal as a diagnosis. Symptoms include irritability, insomnia, appetite loss, restlessness, and cravings. Symptoms peak in the first week and can last two weeks or more.
Memory and attention
THC impairs short-term memory, attention, and reaction time during use. Findings on whether these deficits persist after months of abstinence are mixed. Some studies show small effects. Others show none.
Cannabinoid hyperemesis syndrome
Heavy long-term use can cause repeated episodes of severe vomiting and abdominal pain. Hot showers or baths bring relief, which is a hallmark of the condition. Stopping cannabis stops the episodes. Emergency department visits for this condition rose in states after legalization.
Respiratory effects
Smoked cannabis delivers tar and combustion products. Chronic cough, phlegm, and bronchitis are reported in long-term smokers. A link to lung cancer is not established.
Heart and blood vessels
THC raises heart rate and can drop blood pressure on standing. Case-crossover studies report a higher rate of heart attack in the hour after cannabis use. Stroke and arrhythmia case reports exist, including in younger users with no other risk factors.
Mental health
Daily use of high-THC products is linked to higher rates of psychosis in observational studies. The link is strongest for people with a personal or family history of psychotic disorders and for use that starts in adolescence. Researchers debate how much is cause and how much is shared risk.
Drug interactions
CBD inhibits the liver enzymes CYP3A4 and CYP2C19. It can raise blood levels of warfarin, clobazam, and other drugs. The FDA label for Epidiolex, a purified CBD drug, calls for liver enzyme monitoring. THC and CBD both interact with sedatives and alcohol.
Regulatory status in the US
No cannabis product has FDA approval for neuropathic pain. FDA-approved cannabinoids cover other uses: Epidiolex for seizures, dronabinol and nabilone for chemotherapy-induced nausea and vomiting. Cannabis remains Schedule I at the federal level. Many states allow medical use.
Practical points
- Track dose, route, and pain score in a log. This shows tolerance as it develops.
- Oral products take 1 to 3 hours to peak and last 6 to 8 hours. Inhaled products peak in minutes and last 2 to 3 hours.
- Tell your clinician about all drugs you take, including blood thinners.
- Screen for a personal or family history of psychosis before long-term use.
- Do not stop prescribed pain drugs without medical advice.