What the evidence actually says
Cannabis is one of the most studied non-prescription substances in medicine, and one of the most over-claimed. The 2017 NASEM report, still the cleanest summary, sorted cannabis claims into three buckets: conclusive evidence, moderate evidence, and limited or no evidence.
There is conclusive evidence for chemotherapy-induced nausea and vomiting (oral cannabinoids), chronic pain in adults, and spasticity in multiple sclerosis. There is moderate evidence for short-term sleep onset and certain kinds of nausea. Everything else, including anxiety, PTSD, depression, ADHD, and IBS, is either limited evidence or a signal worth investigating but not yet proven.
That does not mean cannabis fails to help with those things for some patients. It means the research is not yet strong enough to place it in the same column as things that work for nearly everyone. Treat it as a tool worth trying rather than a guaranteed answer.
What it actually helps with, in clinic
Sleep onset: cannabis often helps people fall asleep faster. The catch is that heavy THC use can suppress REM sleep, so long-term high-dose use can mean more hours asleep but less feeling of rest.
Chronic pain: especially neuropathic (nerve) pain and chronic inflammatory pain. The effect size is modest, but for many patients it allows a lower opioid or NSAID dose.
Nausea and appetite: well established for chemotherapy and HIV-related wasting. The mechanism acts directly on the brain's vomiting centre.
Anxiety: this one is biphasic. Low THC with high CBD often reduces anxiety; high THC on its own can sharply increase it. Dose matters more here than for almost any other use.
Spasticity in MS: enough evidence exists that FDA-approved cannabinoid medicines are available for this exact use.
The risks that are real
Cannabis use disorder is a real diagnosis. Roughly 9 percent of people who use cannabis develop dependence at some point, and the figure rises to about 17 percent for those who start as teenagers. The claim that it is not addictive is wrong. It is less physically addictive than alcohol or opioids, but it is psychologically habit forming.
Tolerance builds quickly. Daily users often find the effective dose creeping upward over months. A tolerance break of 3 to 7 days resets most of it.
Cannabinoid hyperemesis syndrome is rare but real in heavy long-term users: cyclic vomiting, abdominal pain, and hot showers as the only relief. The treatment is to stop using cannabis. It is worth knowing it exists.
Psychosis is the headline risk. People with a personal or family history of psychosis, schizophrenia, or bipolar disorder have a meaningfully higher risk of triggering an episode with high-THC cannabis, especially under age 25. This is the single strongest do-not in the whole patient-facing literature.
Who probably should not use cannabis
Pregnant or breastfeeding patients. THC crosses the placenta and can be detected in breast milk for days after a single dose. No amount has been shown to be without risk.
Patients under 25, especially with a family history of mood or psychotic disorders. The brain is still wiring itself, and high-THC products in particular appear to interfere with that wiring in ways that can last.
Patients with severe cardiovascular disease. Inhaled THC briefly raises heart rate and can trigger angina or an arrhythmia in vulnerable people.
Patients on transplant immunosuppressants without their transplant team's sign-off. The interaction with tacrolimus is the highest-risk one in this whole guide.
Patients who have had a bad reaction to cannabis before. Maybe it will be different this time usually turns out to be wrong.
How to spot a problem forming
You are using more than you meant to, on more days a week than you meant to. The dose has crept up. You feel worse on days you do not use than on days you do. You are using it to manage something that was not the original reason, such as boredom, social anxiety, or anger.
Any one of those on its own is not a crisis. Two or three together over a month is a signal worth attention. The fix is rarely stopping forever. It is usually a structured tolerance break (3 to 7 days off, then a firm cap on weekly use) plus an honest conversation with someone you trust.
If that does not reset things, talk to your physician or a pharmacist who works with cannabis patients. There is no shame in that conversation. Cannabis use disorder is treatable, well studied, and almost never the dramatic spiral the news makes it sound like.
The honest frame
Cannabis is a tool. It is a useful one for several real medical problems and a recreational substance for many other people. It is not a magic plant, and it is not a moral failure. Treat it the way you would treat any other prescription: respect the dose, know the interactions, track honestly whether it is actually helping, and be willing to stop if it is not.
The patients who get the most from cannabis are the ones who stay clear-eyed about both the upsides and the downsides. That is the whole point of this site.