How Does Marijuana Affect You?

Medical marijuana is now legal in a majority of states. A small but growing number of states and cities have legalized recreational pot as well. Marijuana still is the most commonly used illegal drug in the U.S. 

Video Transcript

American Cancer Society: “Marijuana and Cancer.”; Mayo Clinic: “Medical marijuana,” “Is medical marijuana legal?”; American Lung Association: “Marijuana and Lung Health.”

AREFA CASSOOBHOY: Medical marijuana. It's in the news as more and more states legalize it. But what is it used for? Studies show that marijuana may help several conditions. It's most commonly prescribed for chemotherapy side effects like nausea, vomiting, or weight loss. Muscle spasms and stiffness caused by multiple sclerosis. Various pain syndromes and seizures. It can be taken several ways. Inhaled, either through smoke or vapor, as an edible like in a cookie or an herbal tea, or as a liquid under the tongue. But marijuana is not without side effects. It can cause dizziness, confusion, and drowsiness. It can worsen some mental illnesses like depression. And like cigarettes, when smoked, it can hurt your lungs. So if you live in a legal state, talk to your doctor. Like any treatment, you can weigh the benefits and risks to determine if medical marijuana is right for you. For WebMD, I'm Dr. Arefa Cassoobhoy.

Marijuana has some well-proven benefits, including relief for long-term pain. But smoking marijuana can have some bad effects on your health, including making breathing problems worse.

The federal ban on marijuana makes it hard to study its effects on humans. For example, very little research exists on edible marijuana.

Key Chemicals

Marijuana comes from the dried flowers of cannabis plants.It has more than 500 chemicals. Cannabis can have a psychoactive -- or mind-altering -- effect on you.

THC: This is the main psychoactive agent in marijuana. Its full name is delta 9-tetrahydrocannabinol. When you smoke cannabis, THC goes from your lungs into your bloodstream and then into your brain. It stimulates the part of your brain that responds to sources of pleasure, like food and sex. That lets loose a chemical called dopamine, which causes the high.

THC’s effects can vary depending on who you are, the potency of the strain, whether you smoke it or eat it, and other things. It can:

  • Give you a relaxed sense of well-being
  • Heighten your senses, like make colors seem brighter
  • Change your sense of time
  • Make you anxious, afraid, or panicked
  • Make you hallucinate

CBD. Also called cannabidiol, this is another well-studied compound. It doesn’t make you high. Instead, it can counteract the effects from THC and bring you down from any paranoia or anxiety. It also has been found to have beneficial uses in treatment the side effects of chemotherapy and treating epilepsy.

Ways to Use It

You can get cannabis into your body in two main ways: smoking and eating.

Smoking. This, along with inhaling (vaping), is the fastest way for marijuana to work. Your bloodstream carries the THC to your brain so quickly that you may start to feel high within seconds or minutes. The amount of THC in your blood typically peaks in about 30 minutes, then tapers off in 1-4 hours.


Ways you can smoke cannabis include:

  • Rolled into a cigarette
  • In a pipe or water pipe, called a bong
  • In a cigar that has been hollowed out and refilled with marijuana, called a blunt
  • In the form of sticky resins that have been drawn from the cannabis plant. Resins often have much higher amounts of THC than regular marijuana.

Eating or drinking. This slows marijuana’s effects because the THC has to go through your digestive system. It may take 30 minutes to 2 hours for you to get high. But it will last longer -- up to 8 hours -- than if you smoked or vaped pot. You can mix cannabis into brownies, cookies, candy, and other foods, or brew it into a tea.

Whether you smoke cannabis or eat it, remember that it can:

  • Heighten alcohol’s effects on your body
  • Interact with medications. For example, it can raise the dangers of bleeding with blood thinners or make some antiviral drugs not work as well.
  • Hurt your concentration and motor skills. It’s dangerous to drive while you’re high.


People have turned to the cannabis plant as medicine for hundreds of years. Researchers have found that cannabis can help with:

  • Ongoing pain (this is the most common use for medical marijuana)
  • Nausea or throwing up from chemotherapy
  • Seizures from epilepsy, Dravet syndrome or Lennox-Gastaut syndrome.
  • Stiff muscles or muscle spasms from multiple sclerosis. The evidence is stronger for self-reported symptoms by people with MS than for improvements measured by experts.

Evidence is limited that marijuana might help with:


Researchers don’t fully understand all the ways cannabis can affect your mind and body. That’s especially true when it comes to children and young adults and their brains.

Mind. Some evidence suggests that marijuana hurts your learning, memory, and attention for 24 hours after use. Evidence is less strong that your mental skills will get worse with long-term marijuana use. Limited evidence shows that marijuana hurts how you do in school or on the job.


Cancer. No link has been found between smoking marijuana and cancers in the lung, head, or the neck. Limited evidence suggests that heavy marijuana use may lead to one type of testicular cancer. Researchers don’t have enough information whether cannabis affects other cancers, including prostate, cervical, and bladder cancers and non-Hodgkin's lymphoma.

Lungs. Regular marijuana use can give you constant coughs and phlegm. They may go away when you stop smoking. It’s unclear if marijuana can lead to asthma or chronic obstructive pulmonary disorder. Cannabis actually helps open the airways at first. But evidence shows that regular marijuana use will make your lungs not work as well.

Mental health. People with schizophrenia and other psychotic disorders may be more likely to use marijuana heavily, about twice a month. Researchers have also found links between cannabis use and bipolar disorder, major depression, and childhood anxiety. What’s hard to untangle is if marijuana use leads to mental illness, or if it’s the other way around.

See More: A Visual Guide to Schizophrenia

Babies and children. Infants born to women who smoke marijuana while pregnant are more likely to be underweight, be born too early, and need neonatal intensive care. But researchers don’t have enough information to say much about how the babies do later in life.

Secondhand smoke. You probably won’t get high by breathing in someone else’s marijuana smoke. Very little THC is released in the air when they breathe out. Chances are very small that secondhand cannabis smoke will cause you to fail a drug test.

WebMD Medical Reference Reviewed by Neil Lava, MD on December 15, 2018


SOURCES: “State Marijuana Laws in 2018 Map.”

National Institute on Drug Abuse: “What is marijuana?” “How does marijuana work?”
“Secondhand Marijuana Smoke?”

American Cancer Society: “Marijuana and Cancer.”

National Academies Press: “The Health Effects of Cannabis and Cannabinoids: The Current State of Evidence and Recommendations for Research.”

Mayo Clinic: “Marijuana,” “Medical marijuana.”

Harm Reduction Journal: “Cannabis and tobacco smoke are not equally carcinogenic.”

© 2018 WebMD, LLC. All rights reserved.