Cannabis: Risks and effects

What is cannabis?

Cannabis (also known as marijuana, pot, weed, reefer, ganja, Mary Jane, and other slang terms) refers to the dried leaves, flowers, and stems from the hemp plant Cannabis sativa. The leaves contain the psychoactive (mind-altering) chemical THC (tetrahydrocannabinol), as well as several other related compounds called cannabinoids. Concentrated THC is found in hashish hash oil, shatter, wax, and dab oil.

Cannabis is usually smoked in cigarettes called "joints" or in pipes. It can also be mixed in food, such as brownies, cookies, chocolate, and many others, or brewed as a tea and consumed as a beverage. Blunts usually contain tobacco and cannabis, which provide the effects of both THC and nicotine together.

Legal status of cannabis

In 2026, the US federal government reclassified some cannabis from Schedule 1 (a drug with no current medical use and a high potential for abuse or addiction) to Schedule 3 (low to moderate potential for abuse or addiction). The reclassification applies to FDA-approved cannabis products and cannabis products authorized by state-licensed cannabis programs. Other cannabis products remain Schedule 1. The legal status of cannabis in the US is rapidly evolving as more states move to legalize medical and recreational use while federal laws continue to create regulatory uncertainty.

Short-term effects of cannabis

The desirable effects of a cannabis “high” include euphoria, relaxation, increased appetite, and possible heightened sensory perception. The undesirable effects include fast heart rate, impaired coordination, difficulty with problem-solving and clear thinking, and forgetfulness. Impaired judgment makes it dangerous to drive after using cannabis.  As with drinking alcohol, cannabis intoxication may increase vulnerability to sexual assault or other criminal acts by impairing awareness, judgment, or the ability to respond to dangerous situations. Psychotic episodes and more serious medical problems are rare but possible.

Smoking, vaping, and edibles: Is there a difference?

When cannabis is smoked or vaped, the THC passes rapidly from the lungs into the bloodstream, which carries the chemical to the brain and other organs throughout the body. Following inhalation, the onset of effects occurs within minutes and typically peaks in 20–30 minutes. Vaping may also lead to serious lung injury with symptoms of persistent cough, shortness of breath, chest pain, and wheezing. There is also a risk of developing a serious and potentially fatal condition called bronchiolitis obliterans, commonly known as “popcorn lung.” Popcorn lung is characterized by damage and scarring to the small structures in the lung and is associated with inhaling flavored vapes.  

When swallowed in food or drink, THC is absorbed more slowly (onset in 30–90 minutes) and lasts longer, with effects often not peaking until 3 hours after ingestion. These times are highly variable depending on the specific circumstances of the exposure.

Long-term health effects of regular cannabis use

Cannabis smoke contains many of the harmful substances found in tobacco smoke, which contribute to conditions like chronic bronchitis. It might also increase the risk of respiratory tract cancer. There is some evidence that long-term cannabis use might contribute to stroke, heart attack, heart rhythm changes, psychosis, and depression.

Cannabinoid hyperemesis syndrome (CHS) occurs in some long-term cannabis users. CHS includes recurring episodes of nausea, vomiting, and abdominal pain. CHS goes away only when the person stops using cannabis completely.

Cannabis use disorder, a type of substance use disorder, occurs in 9–30% of heavy, chronic cannabis users. There are no FDA-approved medications for cannabis use disorder, but a variety of psychosocial interventions, like cognitive behavioral therapy, have shown promise.

Cannabis and teenagers: Does it affect development?

Cannabis use may cause short-term effects on cognition, such as problems with memory and attention. Research suggests that frequent adolescent use is associated with lasting changes in brain development that may contribute to difficulties with information processing, memory, attention, and other cognitive functions later in life.

Some studies of adolescents followed into young adulthood suggest that persistent or increasing heavy cannabis use may be associated with lower educational attainment, lower income, and greater financial strain compared with nonusers. However, further research is needed to better understand these relationships and whether cannabis use directly causes these outcomes.

Is smoking cannabis worse for you than smoking tobacco?

When compared to a tobacco cigarette, a cannabis cigarette results in as much as 5 times more inhaled carbon monoxide and 3 times more inhaled tar. Cannabis smokers inhale more deeply and hold the smoke in their lungs much longer than tobacco smokers. However, tobacco smokers consume far more cigarettes than cannabis smokers.

How long does cannabis stay in your system?

When cannabis is inhaled, THC reaches the bloodstream very quickly and then usually decreases over 3–4 hours as it is absorbed by body tissues. The half-life (time for the amount of THC in the body to be reduced by half) is 1.3 days in occasional users and 5–13 days in heavy users due to the large accumulation of THC. THC can be detected in the urine of heavy users for up to 30 days.

Can you overdose on cannabis?

Acute toxicity from cannabis exposure mostly consists of excessive sedation, loss of coordination, rapid heart rate, and dilated pupils. While death from cannabis exposure alone appears to be very rare, cannabis use in combination with opiates might be making an impact on the rate of opiate deaths.

Secondhand cannabis smoke

Secondhand smoke from cannabis contains the same toxins and irritants as tobacco smoke along with more particulates. In some reports, the THC concentration of secondhand cannabis smoke has been great enough to result in positive drug tests in non-smokers.

Cannabis use during pregnancy and breastfeeding

Meta-analyses (studies that pool data from many other studies) have produced results that show cannabis-exposed babies to be small for their gestational age, to have lower birth weights and lengths, and to have lower Apgar scores as well as higher rates of preterm delivery, major birth defects, and NICU admission. However, the authors of these meta-analyses indicate that the individual studies show a lot of variation in their results. Most experts recommend that women avoid using cannabis during pregnancy.

THC is known to enter breast milk. Studies of the effects of occasional cannabis use on breastfed infants have not established a pattern of detectable effects, but these studies are not definitive. A legal concern is that cannabis exposure can sometimes be identified through urine testing of infants born to mothers suspected of substance use, allowing evidence of exposure to be obtained without directly testing the mother.

Does cannabis interact with medications or alcohol?

Cannabis makes people drowsy. Combining it with other substances that cause sedation will typically produce significantly more sedation than either substance alone. Alcohol is the most common sedative taken with cannabis. Before taking any medication along with cannabis, check the label or pharmacy-applied stickers to see if there is a warning about causing drowsiness, not operating machinery, or not taking it with alcohol. These are signs that taking the medication while using cannabis is likely to cause more sedation than expected from cannabis alone.

One potentially serious drug interaction with cannabis is an increased risk of bleeding in people also taking the anticoagulant (blood thinner) warfarin. Cannabis can also increase the effects of buprenorphine (pain reliever), tacrolimus (immunosuppressant), clozapine (antipsychotic), and methadone (opiate pain reliever). Cannabis might reduce the effects of several medications, including phenytoin (anti-seizure), carbamazepine (anti-seizure), and rifampin (antibiotic).

What to do if someone has a bad reaction to cannabis

If you or someone else has a bad reaction to cannabis, or has questions about a related substance, use the webPOISONCONTROL online tool to get immediate help, or call your poison center at 1-800-222-1222. Whether online or by phone, expert guidance is always free, confidential, and available 24 hours a day.