Cannabis FAQs

What is cannabis?

Marijuana—also called weed, herb, pot, grass, bud, ganja, Mary Jane, and a vast number of other slang terms—is a greenish-gray mixture of the dried flowers of Cannabis sativa. Stronger forms of cannabis include sinsemilla (from specially tended female plants) and concentrated resins containing high doses of cannabis’ active ingredients, including honey-like hash oil, waxy budder, and hard amber-like shatter. These resins are increasingly popular among those who use them both recreationally and medically.

The main psychoactive (mind-altering) chemical in cannabis, responsible for most of the intoxicating effects that people seek, is delta-9-tetrahydrocannabinol (THC). The chemical is found in resin produced by the leaves and buds primarily of the female cannabis plant. The plant also contains more than 500 other chemicals, including more than 100 compounds that are chemically related to THC, called cannabinoids.

How is cannabis used?

Cannabis can be smoked in hand-rolled cigarettes called joints; it is also smoked in pipes, water pipes (sometimes called bongs), or in blunts (cannabis rolled in cigar wraps). Cannabis can also be used to brew tea and, particularly when it is sold or consumed for medicinal purposes, is frequently mixed into foods (edibles) such as brownies, cookies, or candies. Vaporizers are also increasingly used to consume cannabis.

Does cannabis use affect driving?

Cannabis significantly impairs judgment, motor coordination, and reaction time, and studies have found a direct relationship between blood THC concentration and impaired driving ability. Cannabis is the illicit drug most frequently found in the blood of drivers who have been involved in vehicle crashes, including fatal ones. Two large European studies found that drivers with THC in their blood were roughly twice as likely to be culpable for a fatal crash than drivers who had not used drugs or alcohol. However, the role played by cannabis in crashes is often unclear because it can be detected in body fluids for days or even weeks after intoxication and because people frequently combine it with alcohol. Those involved in vehicle crashes with THC in their blood, particularly higher levels, are three to seven times more likely to be responsible for the incident than drivers who had not used drugs or alcohol. The risk associated with cannabis in combination with alcohol appears to be greater than that for either drug by itself.

Several meta-analyses of multiple studies found that the risk of being involved in a crash significantly increased after cannabis use—in a few cases, the risk doubled or more than doubled. However, a large case-control study conducted by the National Highway Traffic Safety Administration found no significant increased crash risk attributable to cannabis after controlling for drivers’ age, gender, race, and presence of alcohol.

Is cannabis addictive?

Cannabis use can lead to the development of problem use, known as a cannabis use disorder, which takes the form of addiction in severe cases. Recent data suggest that 30% of those who use cannabis may have some degree of cannabis use disorder. People who begin using cannabis before the age of 18 are four to seven times more likely to develop a cannabis use disorder than adults.

Cannabis use disorders are often associated with dependence—in which a person feels withdrawal symptoms when not taking the drug. People who use cannabis frequently often report irritability, mood and sleep difficulties, decreased appetite, cravings, restlessness, and/or various forms of physical discomfort that peak within the first week after quitting and last up to 2 weeks. Cannabis dependence occurs when the brain adapts to large amounts of the drug by reducing production of and sensitivity to its own endocannabinoid neurotransmitters.

Cannabis use disorder becomes addiction when the person cannot stop using the drug even though it interferes with many aspects of his or her life. In 2015, about 4 million people in the U.S. met the diagnostic criteria for a cannabis use disorder; 138,000 voluntarily sought treatment for their cannabis use.

Is cannabis a gateway drug?

Some research suggests that cannabis use is likely to precede use of other licit and illicit substances and the development of addiction to other substances. One study found that adults who reported cannabis use during the first wave of the survey were more likely than adults who did not use cannabis to develop an alcohol use disorder within 3 years; people who used cannabis and already had an alcohol use disorder at the outset were at greater risk of their alcohol use disorder worsening.

Early exposure to cannabinoids in adolescent rodents decreases the reactivity of brain dopamine reward centers later in adulthood. To the extent that these findings generalize to humans, this could help explain the increased vulnerability for addiction to other substances of misuse later in life that most epidemiological studies have reported for people who begin cannabis use early in life. It is also consistent with animal experiments showing THC’s ability to “prime” the brain for enhanced responses to other drugs. For example, rats previously administered THC show heightened behavioral response not only when further exposed to THC but also when exposed to other drugs such as morphine—a phenomenon called cross-sensitization.

These findings are consistent with the idea of cannabis as a “gateway drug.” However, the majority of people who use cannabis do not go on to use other, “harder” substances. Also, cross-sensitization is not unique to cannabis. Alcohol and nicotine also prime the brain for a heightened response to other drugs and are, like cannabis, also typically used before a person progresses to other, more harmful substances.

It is important to note that other factors besides biological mechanisms, such as a person’s social environment, are also critical in a person’s risk for drug use. An alternative to the gateway-drug hypothesis is that people who are more vulnerable to drug-taking are simply more likely to start with readily available substances such as cannabis, tobacco, or alcohol, and their subsequent social interactions with others who use drugs increases their chances of trying other drugs. Further research is needed to explore this question.

Can secondhand cannabis smoke cause a “contact high” or a failed drug test?

To answer this question, researchers measured the amount of THC in the blood of non-smokers who spent 3 hours in a well-ventilated space with people casually smoking cannabis. THC was present in the blood of the non-smokers, but the amount was well below the level needed to fail a drug test.

When the non-smokers were exposed for an hour to high-THC cannabis (11.3% THC concentration) in an unventilated room, they reported mild effects (a “contact high”) and displayed mild impairments on performance in motor tasks. In the hours directly following exposure, their urine tested positive for cannabis.

What are the health effects of secondhand cannabis smoke?

The known health risks of secondhand exposure to cigarette smoke—to the heart and lungs, for instance—raise questions about whether secondhand exposure to cannabis smoke poses similar health risks, but very little research on this question has been conducted at this point.

A 2016 study in rats found that secondhand exposure to cannabis smoke affected a measure of blood vessel function as much as secondhand tobacco smoke, and the effects lasted longer. This research has not yet been conducted with human subjects, but the toxins and tar levels known to be present in cannabis smoke raise concerns about secondhand smoke exposure among vulnerable populations, such as children and people with asthma.

Is cannabis safe to use during pregnancy?

Research shows that pregnant women who use cannabis have a 2.3 times greater risk of stillbirth. Animal studies show an increased risk of miscarriage when cannabis is used early in pregnancy, although there is no human research to connect this.

More research is needed on how cannabis use during pregnancy could impact fetal health, but given the potential of cannabis to negatively impact the developing brain, women should refrain from using cannabis while trying to get pregnant, during pregnancy, and while breastfeeding. Some women use cannabis to treat severe nausea during pregnancy; however, there is no research confirming that this is a safe practice, and it is generally not recommended. Women considering using medical cannabis while pregnant should not do so without checking with their health care providers.

How does prenatal exposure to cannabis affect a child's development?

Human research has shown that some babies born to women who used cannabis during pregnancy display altered responses to visual stimuli, increased trembling, and a high-pitched cry, which could indicate problems with neurological development. Evidence is mixed as to whether cannabis use by pregnant women is associated with low birth weight or premature birth, although long-term use may elevate these risks.

Associations have been found between cannabis use during pregnancy and future developmental and hyperactivity disorders in children. In school, cannabis-exposed children are more likely to show gaps in problem-solving skills, memory, and the ability to remain attentive. (More research is needed to disentangle cannabis-specific effects from those of other environmental factors.)

Prenatal cannabis exposure is also associated with an increased likelihood of a person using cannabis as a young adult, even when other factors that influence drug use are considered.

Is it safe to use cannabis while breastfeeding?

Very little is known about cannabis use and breastfeeding. One study suggests that moderate amounts of THC find their way into breast milk when a nursing mother uses cannabis. Some evidence shows that exposure to THC through breast milk in the first month of life could result in decreased motor development at 1 year of age. There have been no studies to determine if exposure to THC during nursing is linked to effects later in the child’s life. With regular use, THC can accumulate in human breast milk to high concentrations. Because a baby’s brain is still forming, THC consumed in breast milk could affect brain development.

Given all these uncertainties, nursing mothers are discouraged from using cannabis. New mothers using medical cannabis should be vigilant about coordinating care between the doctor recommending their cannabis use and the pediatrician caring for their baby.