How Marijuana Works

By: Kevin Bonsor & Nicholas Gerbis
A sticker in San Francisco supports the passage of California's prop 19 to legalize marijuana in 2010. See more controlled substance pictures.
© Mike Blake/Reuters/Corbis

Ganja. Bud. Herb. Call it what you will, there's no question that marijuana has become a divisive issue in U.S. politics. Whether for its role in supporting drug cartels, its tendency to worsen the impact of unbalanced drug laws, or simply its position in the center of the drug legalization movement, the sticky icky has become a rather sticky wicket.

Although banned by the U.S. government in 1937, marijuana has been the subject of much legal wrangling since the 1980s, gradually opening the way for medicinal use in 20 states and the District of Columbia as of January 2014. Colorado and Washington also have approved recreational use. Meanwhile, more than a dozen states have decriminalized holding of small quantities of weed [sources: Rabin, Cassidy].


Even though it's celebrated on 4/20 each year and support for legalization has doubled in the past 20 years or so, usage has remained relatively flat [source: Saad]. According to a July 2013 Gallup poll, roughly 38 percent of Americans had tried marijuana, and 7 percent, or 22 million people, partook of the drug. The United States drug market is one of the world's most commercially viable and attracts drug traffickers from every corner of the globe. Depending on location and quality of product, marijuana can cost anywhere from $1.77-$17.66 per gram ($800-$8,000 per pound) [source: Wilson].

Typically, marijuana refers to the buds and leaves of the Cannabis sativa or Cannabis indica plants. The cannabis plant contains more than 500 chemicals, including delta-9-tetrahydrocannabinol (THC), the main psychoactive chemical [source: Mehmedic et al.]. Cannabis impairs memory during and between uses, although the restorative effects of long-term abstinence remain unclear. In addition, during use, it adversely affects motor coordination, attention and judgment, increases heart rate and raises levels of anxiety. Studies also show that marijuana smoke contains irritants and cancer-causing chemicals typically associated with cigarettes [sources: Hall and Solowij; NIDA]. In this article, you will learn about marijuana, why this drug is so popular and what effects it has on your mind and body.

The Marijuana Plant

The leaves and flower head of Cannabis sativa
© Carol Sharp/ Ubiquitous/Corbis

The history of marijuana use reaches back farther than many would guess. Cultivation of the plant dates back thousands of years. The first written account of cannabis cultivation (ostensibly used as medical marijuana) is found in Chinese records dating from the 28th century B.C.E. And a nearly 3,000-year-old Egyptian mummy contained traces of THC, the main psychoactive chemical in marijuana, as well as other drugs [source: Parsche and Nerlich].

Cannabis sativa is perhaps the world's most recognizable plant. Pictures of the ubiquitous, green cannabis leaf show up in the news media, textbooks and drug-prevention literature. Its shape is made into jewelry, plastered on bumper stickers and clothing, and spray-painted on walls. The leaves are arranged palmately, radiating from a common center, like the fingers of a hand spreading apart. Although most people know what the cannabis plant looks like, they may know very little about its horticulture.


Believed to be a native plant of India, Cannabis sativa possibly originated in a region just north of the Himalayas. It's an herbaceous annual that can grow to a height of 8-12 feet (4 to 5.4 meters), with reports as high as 20 and 25 feet (the indica variety tends to be squatter). The plant has flowers that bloom from late summer to mid-fall, when grown outdoors. Of course, plenty of people grow cannabis indoors, too. (See How Grow Houses Work for more information.)

Cannabis plants usually have one of two types of flowers, male or female, and some plants have both. Male flowers grow in elongated clusters along the leaves and turn yellow and die after blossoming. Female flowers grow in spike-like clusters and remain dark green for a month after blossoming, until the seed ripens. Hashish, which is more potent than marijuana, is made from the resin of the cannabis flowers.

As we mentioned, marijuana plants contain hundreds of chemicals, 109 of which fit into a category called cannabinoids [source: Mehmedic et al.]. THC, of course, is one of these cannabinoids, and it's the chemical most often associated with the effects that marijuana has on the brain. The concentration of THC and other cannabinoids varies depending on growing conditions, plant genetics and processing after harvest. You'll learn more about the potency of THC and the toxicity of marijuana next.

Marijuana in the Body

© 2001

Every time someone smokes a marijuana cigarette or ingests marijuana in some other form, THC (delta-9-tetrahydrocannabinol) and other chemicals enter the user's body. The chemicals make their way through the bloodstream to the brain and then to the rest of the body. The most powerful chemical in marijuana is THC, which is primarily responsible for the "high" associated with the drug.

The most common way to use marijuana is to smoke it (more on how people do that later). Smoking is also the most expedient way to get the THC and other chemicals into the bloodstream. When a person inhales smoke from marijuana, the THC goes directly to the lungs. Your lungs are lined with millions of alveoli, the tiny air sacs where gas exchange occurs. These alveoli have an enormous surface area -- much greater than that of your skin -- so they make it easy for THC and other compounds to enter the body. The lungs absorb the smoke just seconds after inhalation. Vaporizers, which heat the cannabis sufficiently to vaporize its active ingredients without burning it, provide an inhalable alternative to smoking.


You can also eat marijuana. In this case, the marijuana enters the stomach, and the blood absorbs it there. The blood then carries it to the liver and the rest of the body. The stomach absorbs THC more slowly than the lungs. When users eat marijuana, the levels of THC in the body are lower, but the effects last longer. Some people who use marijuana therapeutically drink it in tea, but the process tends to dilute the amount of THC available for absorption [source: Gieringer and Rosenthal]. Since Colorado voted to ease its marijuana laws, the state has seen an uptick in the number of kids accidentally ingesting pot brownies, cookies, candies and drinks [source: Kelly].

Marijuana and the Brain

© 2001

After you inhale marijuana smoke, its chemicals zip throughout the body. THC is a very potent chemical compared to other psychoactive drugs. Once in your bloodstream, THC typically reaches the brain seconds after it is inhaled and begins to go to work.

Marijuana users often describe the experience of smoking the drug as initially relaxing and mellow, creating a feeling of haziness and light-headedness (although those feelings may differ depending on the particular strain). The user's eyes may dilate, causing colors to appear more intense, and other senses may be enhanced. Later, the user may have feelings of paranoia and panic. The interaction of the THC with the brain is what causes these feelings. To understand how marijuana affects the brain, you need to know about the parts of the brain that are affected by THC. Here are the basics:


  • Neurons are the cells that process information in the brain.
  • Chemicals called neurotransmitters allow neurons to communicate with each other.
  • Neurotransmitters fill the gap, or synapse, between two neurons and bind to protein receptors, which allow various functions in the brain and body to be turned on and off.
  • Some neurons have thousands of receptors that are specific to particular neurotransmitters.
  • Foreign chemicals, like THC, can mimic or block actions of neurotransmitters and interfere with normal functions.

Your brain has groups of cannabinoid receptors concentrated in several different places (see picture). These cannabinoid receptors can affect the following mental and physical activities:

  • Short-term memory
  • Coordination
  • Learning
  • Problem-solving

Cannabinoid receptors are activated by a neurotransmitter called anandamide. Like THC, anandamide is a cannabinoid, but one that your body makes. THC mimics the actions of anandamide, meaning that THC binds with cannabinoid receptors and activates neurons, which causes adverse effects on the mind and body.

High concentrations of cannabinoid receptors exist in the hippocampus, cerebellum and basal ganglia. The hippocampus sits within the temporal lobe and is important for short-term memory. When the THC binds with the cannabinoid receptors inside the hippocampus, it interferes with the recollection of recent events. THC also affects coordination, which the cerebellum controls. The basal ganglia direct unconscious muscle movements, which is another reason why motor coordination is impaired when under the influence of marijuana.

Other Physiological Effects of Marijuana

Along with the brain, the side effects of marijuana reach many other parts of the body. Overall, users may notice the following short-term side effects:

  • Problems with memory and learning
  • Distorted perception
  • Difficulty with thinking and problem solving
  • Loss of coordination
  • Increased heart rate
  • Anxiety, paranoia and panic attacks

The initial effects created by the THC in marijuana wear off after an hour or two, but the chemicals stay in your body for much longer. The terminal half-life of THC can range from about 20 hours to 10 days, depending on the amount and potency of the marijuana used. This means that if you take 1 milligram of THC that has a half-life of 20 hours, you will still have 0.031 milligrams of THC in your body more than four days later. The longer the half-life, the longer the THC lingers in your body.


The debate over the addictive capacity of marijuana continues. Ongoing studies now show a number of possible symptoms associated with the cessation of marijuana use. These symptoms most commonly include irritability, nervousness, depression, anxiety and even anger. Other symptoms are restlessness, severe changes in appetite, violent outbursts, interrupted sleep or insomnia. In addition to these possible physical effects, psychological dependence usually develops because a person's mind craves the high that it gets when using the drug.

Beyond these effects, marijuana smokers are susceptible to the same health problems as tobacco smokers, such as bronchitis, emphysema and bronchial asthma. Other effects include dry mouth, red eyes, impaired motor skills and impaired concentration. Long-term use of the drug can increase the risk of damaging the lungs and reproductive system, according to the U.S. Drug Enforcement Agency (DEA). It has also been linked to heart attacks.

Although marijuana is known to have negative effects on the human body, there's a raging debate over the use of medicinal marijuana. Some say that marijuana should be legalized for medical use because it has been known to suppress nausea, relieve eye pressure, decrease muscle spasms, stimulate appetite, stop convulsions and eliminate menstrual pain. Because of its therapeutic nature, marijuana has been used in the treatment of several conditions:

  • Cancer and AIDS (to suppress nausea and stimulate appetite)
  • Glaucoma (to alleviate eye pressure)
  • Epilepsy (to stop convulsions)
  • Multiple sclerosis (to decrease muscle spasms)

Others believe the negative effects of marijuana usage outweigh the positive. As of February 2014, in contrast to moves by several states toward medical marijuana statutes, the U.S. Drug Enforcement Agency listed marijuana as Schedule 1 substance. Schedule 1 is reserved for "the most dangerous drugs," substances with "no currently accepted medical use and a high potential for abuse... [and] potentially severe psychological or physical dependence," including heroin, LSD and Ecstasy.

Marijuana Potency

Whether marijuana is more potent today than it was 30 or 40 years ago is at the center of much debate. The U.S. federal government has said that the levels of potency have risen anywhere from 10 to 25 times since the 1960s. Is this a myth or reality?

"There's no question that marijuana, today, is more potent than the marijuana in the 1960s. However, if you were to look at the average marijuana potency which is about 3.5 percent, it's been relatively stable for the last 20 years. Having said that, it's very important that what we have now is a wider range of potencies available than we had in the 1970s, in particular."


That's from Alan Leshner, the Director of the National Institute on Drug Abuse, while he was testifying in front of the U.S. House Subcommittee on Crime in 1999.

Those who support the legalization of marijuana say that the data are skewed because testing was only performed on marijuana of specific geographic origins in the 1960s and 1970s, and therefore isn't representative of marijuana potency overall. Officials obtained the samples from a type of Mexican marijuana that is known to contain low levels of THC -- 0.4 to 1 percent [source: Kuhn et al.]. When these levels are compared to other types of marijuana, it looks as if potency levels have risen in the last 30 years.

Typical THC levels, which determine marijuana potency, range from 0.3 to 4 percent. However, some specially grown plants can contain THC levels as high as 25 percent, leading to a call by some users for producers to put out mellower Mary Jane [source: Marris]. Several factors are involved in determining the potency of a marijuana plant, including:

  • Growing climate and conditions
  • Plant genetics
  • Harvesting and processing
  • Desire by small growers to maximize profit

The time at which the plant is harvested affects the level of THC. Additionally, female varieties have higher levels of THC than male varieties.

As a cannabis plant matures, its chemical composition changes. During early development, cannabidiolic acid is the most prevalent chemical. Later, cannabidiolic acid is converted to cannabidiol, which is later converted to THC when the plant reaches its floral maturation.

To determine the average potency levels of marijuana, researchers need to examine a cross section of cannabis plants, which wasn't done in the 1960s and 1970s. This makes it difficult to make accurate comparisons between the THC levels of that period and the THC levels of today. Moreover, establishing a clear relationship between THC levels and impairment is not as straightforward as with, say, blood alcohol content.

Marijuana Usage

An industrious grower

People typically use marijuana in one of the following different ways:

  • Cigarette -- Dried marijuana buds are rolled into a cigarette, also called a joint.
  • Cigar -- Slice open a cigar, remove the tobacco and refill it with marijuana. Often called a blunt.
  • Pipe – Tobacco pipes are also used to smoke marijuana.
  • Bong -- Water pipes, typically with a long tube rising out of a bowl-shaped base, trap smoke until it's inhaled, raising the amount of THC taken in.
  • Food -- Marijuana is sometimes baked into foods, such as brownies, or brewed as tea.

With millions of users, marijuana use isn't limited to one demographic. However, marijuana use is highest among younger people. Marijuana use in teenagers doubled from 1992 to 1999, when one out of every 13 kids aged 12 to 17 was a current user. In 1998, according to the National Center on Addiction and Substance Abuse (NCASA), half of all 13-year-olds said that they could find and purchase marijuana, and 49 percent said that they had first tried marijuana at age 13 or younger.


Since that time, emergency room mentions of marijuana among 12- to 17-year olds have jumped 48 percent, possibly due to the drug's increased potency. In 2004, NCASA reported that children and teens were "3 times likelier to be in treatment for marijuana than for alcohol, and 6 times likelier to be in treatment for marijuana than for all other illegal drugs combined" [source: NCASA]. Following the teenage years, young adults are the most likely demographic to regularly toke up, although the percentage of people who have tried it remains higher among baby boomers and their children [source: Saad].

As of January 2014, 20 U.S. states have decriminalized or legalized marijuana for medical purposes. Colorado and Washington have legalized growing and possessing small amounts of the drug for recreational use, with Florida, New York and other states considering looser laws. Meanwhile, there is evidence of backlash in some counties and towns, which are moving to ban sales within their boundaries, much as some counties are dry [sources: Johnson; Reuters]. In places where possession and/or use remain illegal, the following factors typically affect sentencing.

  • Quantity - Penalties vary based on the amount of marijuana found in the person's possession.
  • Selling - Penalties are more severe for those intending to sell marijuana.
  • Growing - Penalties are also more severe for those cultivating cannabis.
  • Location - A person arrested for selling marijuana near a school will often face harsher penalties.

(There's a state-by-state guide to facts about marijuana penalties at NORML.)

Jail sentences and fines have done little to suppress the use of marijuana in the United States. Despite health and legal risks, pot continues to be the illegal drug of choice for many Americans.

Lots More Information

Related Articles

More Great Links

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  • Gieringer, Dale and Ed Rosenthal. "Marijuana Medical Handbook: Practical Guide to Therapeutic Uses of Marijuana." Quick American Archives. Oct. 28, 2008. (Jan. 31, 2014)
  • Hall, Wayne and Nadia Solowij. "Adverse Effects of Cannabis." The Lancet. Vol. 352. Page 1611. Nov. 14, 1998. (Jan. 31. 2014)
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