What are the side effects of cannabis?

Short-term side effects (typical at standard doses): dry mouth, red eyes, increased appetite, slowed reaction time, impaired short-term memory, altered time perception, drowsiness, and increased heart rate. Less common: anxiety, paranoia, dizziness, nausea, "greening out" (acute overconsumption episode). Long-term concerns: cannabis use disorder (~9% of adult users), cardiovascular risk, adolescent cognitive effects, and possible cannabinoid hyperemesis syndrome with heavy chronic use.

How long do cannabis side effects last?

Most acute side effects resolve within hours of the high subsiding — typically 2-4 hours after smoking or vaping, 4-12 hours after edibles. The "greening out" overconsumption episode resolves over 1-6 hours with hydration, food, and rest. Cannabis use disorder, cardiovascular effects, and adolescent cognitive effects are long-term issues that develop with chronic use.

Can you overdose on cannabis?

There is no documented fatal overdose threshold for cannabis in adults. However, you can overconsume — taking more than your body and tolerance can comfortably handle, producing severe anxiety, paranoia, vomiting, dizziness, and dissociation. This is uncomfortable and frightening but not life-threatening. Edibles are the most common cause because the slow onset (30-120 min) leads users to redose before the first dose has fully kicked in.

Is smoking weed bad for your lungs?

Chronic cannabis smoking is associated with bronchitis, chronic cough, and increased mucus production due to combustion byproducts. Unlike tobacco, long-term cannabis smoking has not consistently been linked to elevated lung cancer risk in research, but the evidence base is limited. Vaporization avoids combustion and substantially reduces respiratory exposure, though it is not risk-free (see EVALI outbreak from unregulated vape cartridges).

Who should NOT use cannabis?

Pregnant or breastfeeding women, adolescents and young adults (especially before age 25), people with personal or family history of psychotic disorders, people with significant cardiovascular disease, and people on prescription medications metabolized by CYP450 enzymes (warfarin, certain antidepressants, antiepileptics) without consulting a pharmacist first. Cannabis use disorder is also more common in people with prior substance use disorders.

What is "greening out"?

Greening out is a temporary cannabis overconsumption episode with sweating, pallor, dizziness, anxiety, nausea, and sometimes vomiting. Caused by THC dose exceeding the user's tolerance — most common with edibles (slow onset prompts redosing) and concentrate dabbing. Not life-threatening; resolves over 1-6 hours. Best response: hydration, food, calm environment, and time.

Cannabis Side Effects — Short-Term and Long-Term

Medical Disclaimer: This information is educational only and does not constitute medical advice. Cannabis affects everyone differently. Always consult a qualified healthcare provider before using cannabis, especially if you take other medications. Never discontinue prescribed medications without medical supervision.

Short-Term Side Effects

These appear during or shortly after acute cannabis use and resolve within hours of the high wearing off. Severity is dose-dependent — mild at low doses, more pronounced at high doses or in inexperienced users.

Common (most users will experience these at typical doses)

Dry mouth ("cottonmouth") — cannabinoids inhibit saliva production via CB1 receptors in the salivary glands. Drink water.
Red, dry eyes — THC dilates ocular blood vessels and reduces intraocular pressure (the latter being the basis of older glaucoma research).
Increased appetite ("the munchies") — THC stimulates the hypothalamus and the body’s ghrelin/leptin signaling.
Slowed reaction time and impaired short-term memory — CB1 activation in the prefrontal cortex and hippocampus.
Altered time perception — minutes can feel like much longer.
Drowsiness — particularly with indica-leaning chemovars and at higher doses.
Increased heart rate (tachycardia) — THC raises resting heart rate by 20–50% acutely. Returns to baseline as the high subsides.

Less common but possible

Anxiety, paranoia, or panic — especially at high doses, in inexperienced users, or in unfamiliar settings. The biphasic effect: low THC doses can be calming, high doses anxiogenic.
Dizziness or lightheadedness — THC can drop blood pressure when standing up suddenly (orthostatic hypotension).
Nausea or vomiting — paradoxical, since THC is also antiemetic at lower doses. More common with edibles overdose.
"Greening out" — a temporary acute overconsumption episode with sweating, pallor, dizziness, anxiety, and nausea. Resolves over 1–6 hours with hydration, food, calm environment.
Acute psychotic-like symptoms — rare but real at very high doses or in vulnerable individuals: depersonalization, derealization, paranoid thinking. Usually resolves with the high.

How to Manage Acute Side Effects

Most acute side effects pass within a few hours. Practical management:

Hydrate — addresses dry mouth and supports the body’s clearance.
Eat something — particularly helpful for nausea and lightheadedness; the “greening out” episode often resolves faster with food.
Find a calm, low-stimulation space — reduces anxiety and paranoia; lying down is fine.
CBD can dampen THC overdose — some research supports this, since CBD is a negative allosteric modulator of CB1.
Black peppercorns — folk remedy with some plausible mechanism (beta-caryophyllene is a CB2 agonist). Worth trying if available.
Sleep through it — the most reliable solution if you can.
When to call 911 or go to the ER — if symptoms persist beyond 6–8 hours, if there is chest pain, seizure-like activity, or sustained psychotic symptoms, or if you suspect another substance is involved (synthetic cannabinoids, contaminated product). Cannabis itself is not lethal in adults at any documented dose, but treatable medical conditions can present similarly to a bad high.

Long-Term Side Effects

Cannabis Use Disorder (CUD)

Approximately 9% of adult cannabis users develop cannabis use disorder — a clinical diagnosis defined by DSM-5 criteria including impaired control, social impairment, risky use, and pharmacological criteria. The rate rises to about 17% for people who start in adolescence. CUD is real, treatable, and underdiagnosed.

See our dedicated page on Cannabis Use Disorder — Signs & Support, or visit our companion site CannabisDependence.org for evidence-based help.

Cardiovascular Risk

Acute THC use raises heart rate and is a measurable acute cardiovascular stressor. Chronic use has been linked in observational studies to elevated risk of heart attack and stroke, particularly in people with underlying cardiovascular disease. A 2025 JAMA study found significant cardiovascular event association with daily cannabis use after adjusting for confounders. See Cannabis & Cardiovascular Risks for the full evidence review.

Cognitive Effects in Adolescents

Cannabis use during adolescence (typically defined as before age 25, when the prefrontal cortex finishes developing) is associated with lasting cognitive effects: lower IQ, executive function deficits, increased risk of cannabis use disorder, and elevated risk of psychotic disorders in genetically vulnerable individuals. The evidence for adolescent cognitive effects is among the strongest in cannabis epidemiology. Adult-onset use shows much milder cognitive effects, most of which reverse with abstinence.

Mental Health

Cannabis use is associated with elevated risk of psychotic disorders (particularly in people with family history of schizophrenia), worsened anxiety and depression in some users, and increased risk of cannabis-induced psychosis in heavy users of high-potency products. The relationship is bidirectional and complex — cannabis can both cause and be used to self-treat mental health conditions, often in the same person. See Cannabis & Mental Health.

Respiratory Effects (Smoking and Vaping)

Smoked cannabis exposes the lungs to combustion byproducts: tar, carbon monoxide, and over a hundred identifiable compounds, many of which are also in tobacco smoke. Chronic cannabis smoking is associated with bronchitis, chronic cough, and increased mucus production. Unlike tobacco, the long-term cannabis-smoking literature has not consistently shown elevated lung cancer risk — but the studies are limited and the question is not closed.

Vaporization avoids combustion and substantially reduces respiratory exposure but is not risk-free. Vaping cartridges from unregulated sources caused the EVALI outbreak (2,807 hospitalizations, 68 deaths in 2019–2020), traced to vitamin E acetate as a cutting agent in illicit THC vapes. Stick to regulated dispensary products.

Cannabinoid Hyperemesis Syndrome (CHS)

Heavy long-term cannabis users can develop CHS — a paradoxical syndrome of cyclical vomiting, abdominal pain, and compulsive hot showers (the only thing that brings relief). CHS resolves with cannabis cessation but can take weeks to fully clear. Estimated to affect ~3% of heavy users; underdiagnosed because patients (and clinicians) don’t connect the symptoms to cannabis.

Population-Specific Risks

Pregnancy and Breastfeeding

THC crosses the placenta and is excreted in breast milk. Maternal cannabis use during pregnancy is associated with low birth weight, increased risk of preterm delivery, and possible long-term neurodevelopmental effects in the offspring. Cannabis is not recommended during pregnancy or breastfeeding. See Cannabis & Pregnancy.

Adolescents and Young Adults

The developing brain is more vulnerable to lasting cannabis effects than the adult brain. Younger users are at higher risk of CUD, cognitive effects, and psychotic disorders. Cannabis is not recommended before age 21, and ideally not before 25 (when prefrontal development completes).

People with Mental Health History

Cannabis can worsen anxiety, depression, and psychotic disorders in some users. Personal or family history of psychosis is a meaningful contraindication for high-THC cannabis. Low-dose CBD-dominant products may still be appropriate but should be discussed with a psychiatrist.

People on Prescription Medications

Both THC and CBD interact with the CYP450 liver enzyme system, changing blood levels of many common medications. Critical interactions include warfarin, certain antidepressants, antiepileptics, and immunosuppressants. Talk to a pharmacist before combining cannabis with prescription drugs. See Cannabis Drug Interactions.

People With Cardiovascular Disease

Acute cannabis use raises heart rate and the “double product” (heart rate × blood pressure), which stresses the heart. People with coronary artery disease, recent heart attack, uncontrolled hypertension, or arrhythmia should avoid cannabis or use only low-dose CBD-dominant products under medical guidance.

Driving

Cannabis impairs reaction time, lane control, and judgment. Driving impairment after a single dose can persist 4–8 hours, longer for edibles. Do not drive after using cannabis. See Driving & Cannabis Impairment.

Comparing Risk to Other Substances

Multiple expert assessments (Nutt et al. 2010 Lancet; the 2017 NASEM report; comparative drug-harm scoring) place cannabis below alcohol, opioids, tobacco, and methamphetamine on overall harm metrics — counting both individual and societal harm. This does not mean cannabis is harmless; it means relative risk is lower than commonly perceived. See Cannabis vs. Alcohol and Cannabis vs. Other Substances.

The Bottom Line

Most cannabis side effects are mild, predictable, and resolve quickly. The serious long-term concerns — cannabis use disorder, cardiovascular risk, adolescent cognitive effects, and mental health interactions — are real and worth taking seriously. The biggest risk-mitigation steps: start low, go slow, avoid daily use if you can, don’t drive, and talk to a clinician if you take any prescription medications or have a history of cardiovascular or mental health conditions.

If you’re experiencing cannabis-related side effects that aren’t resolving, or if you suspect you may have CUD: see Cannabis Use Disorder on this site, or visit CannabisDependence.org.