Updated on January 21, 2020.
Medical content reviewed by Dr. Joseph Rosado, MD, M.B.A, Chief Medical Officer
The ongoing movement to legalize medical marijuana is rolling along in the US, but there may be a bit of catch-up needed when it comes to side effect and warning labels. At the moment, none of the states that have legalized marijuana for medicinal or recreational use have clear warning labels about possible side effects like those found on tobacco.
That said, medical marijuana can have side effects you should know about. We’re mainly talking about THC-containing cannabis here, which can be smoked, vaped, or eaten, not THC-free CBD products — although those can have side effects too.
The long-term effects of THC-containing treatments are still being sussed out by researchers and are worthy of their own article. But here we’ll talk about the well-documented short-term side effects. Keep in mind that the impact can vary based on the amount you use, the method of delivery, and your medical condition. It’s best to talk to your physician about your own individual risks of side effects.
Dizziness/lightheadedness. This can be a side effect of any medical marijuana product that contains THC, or tetrahydrocannabinol, the active ingredient in cannabis. THC is a cannabinoid, a chemical that can bind to the body’s naturally occurring endocannabinoid receptors. These receptors are found in high concentrations in some parts of the brain and at lower levels elsewhere in the brain and body. THC can cause a temporary rise in blood pressure but also a drop when you stand up, known as orthostatic hypotension, that can make you dizzy. THC can also increase your sensory perception, magnifying sights, sounds, and smells. This may also make it harder for the brain to focus on maintaining balance. Talk to your doctor about what to do in your situation, particularly if you have a medical condition that also affects balance such as multiple sclerosis or Parkinson’s disease. But in general you can try decreasing your dose, using a form of cannabis that has a more gradual effect on the body (say edibles, instead of smoking it), or by switching to a strain that has less THC. Generally it’s a good idea to be prepared for possible dizziness before using a product. And new users are more likely to have dizziness than long-term users, so the body may adjust over time.
Drowsiness. This is another side effect that can be related to THC binding to endocannabinoid receptors in the brain. Marijuana also contains another substance, called myrcene, that relaxes muscles and can have a sedative effect, and may also enhance the already existing sedating effects of THC. Certain types of marijuana, such as Indica strains, may be more likely to cause drowsiness compared with other strains. Some delivery methods, such as edibles and patches, may cause drowsiness more than others. If switching delivery methods or strains doesn’t help alleviate this side effect, you can try using a marijuana medicine at bedtime. It’s a good idea to use the same precautions as you would with any other sedating medication and avoid operating machinery or driving while using the drug.
Paranoia: Along with other side effects, cannabis can affect mental health, which may be more likely in people with existing mental health issues or those who are vulnerable to them. A 2015 randomized placebo-controlled trial in 121 people found that intravenous THC increased paranoid thoughts in people who were already prone to them. The study participants had had at least one paranoid thought in the previous month but did not have a mental illness. In the study, 50% of people reported paranoid thoughts when treated with THC compared with 30% of people treated with a placebo. THC was also more likely to be linked to a lower mood, anxiety, and short-term memory impairment compared to placebo. These are generally short-term effects and won’t be a problem for everyone. However, cannabis use is typically contraindicated in those who are prone to psychosis or who have severe personality disorders.
Slowed reaction time: Another side effect of marijuana is a slowed reaction time. This may be due to THC’s impact on the thalamo-cortico-striatal circuit, a brain network linked to the perception of time. It’s one of the reasons that time seems to slow down when using marijuana. THC can also alter blood flow the cerebellum, a region of the brain that helps with coordination and movement. Again, talk to your doctor and consider switching to a strain with lower THC to see if that helps. And slowed reaction time is another good reason to listen to your doctor’s advice to avoid driving while using this medication.
Increased heart rate: Marijuana can affect the cardiovascular system too. In addition to affecting blood pressure, it can increase heart rate and make your heart work harder. In a 2018 study of 17 people, heart rate peaked 30 minutes after participants consumed marijuana, either smoked or vaporized, and returned to normal within three to four hours. The effect was an extra 19 beats per minute, on average. It was more pronounced with a higher than a lower dose and with vaporization compared with smoking. Although marijuana can affect your heart rate, a 2017 review of data by the National Academies of Sciences, Engineering, and Medicine found that there is “limited evidence” that short-term use can trigger a heart attack in people at risk and “no evidence” one way or another that chronic use is linked to heart attack risk. That said, have a conversation with your doctor about your risks from this particular side effect. Some delivery methods and doses may affect the heart less than others.
Red eyes Cannabis can make your eyes red, and it’s not due to the irritation from smoke. Because THC can dilate or expand blood vessels, this can also affect your eyes. As blood flow increases, the result is more pronounced blood vessels in the sclera or whites of your eyes. In a 2018 study of 17 people in the journal JAMA network, smoking or vaping cannabis was more likely to be associated with eye redness compared with placebo. Vaping resulted in more THC in the blood than smoking and was linked to more side effects — like red, dry eyes — than smoking cannabis. So what can you do about it? The good news is that there are home remedies for bloodshot eyes, including using cold compresses to shrink blood vessels. While some over-the-counter drops promise to reduce redness, they can contain decongestants that acts as vasoconstrictors to shrink blood vessels. Overuse of such drops can lead to a rebound reactions, which means eyes may be redder when the drops wear off, and persistent redness of the eyes if you don’t use them. For bloodshot eyes in general, you should try lubricating eye drops, also known as artificial tears, which do not contain decongestants, according to the American Academy of Ophthalmology.
Dry mouth Dry mouth is a common side effect of cannabis use and may be due to THC binding with receptors that affect salivary flow. About 10% to 25% of people report dry mouth as a side effect. That’s the most common side effect after dizziness, which is reported by 30% to 60% of people. To reduce your chance of dry mouth, drink enough fluids to avoid dehydration. You can also try mouthwashes and rinses specifically formulated to combat dry mouth, which you can buy over the counter. Another option? Try chewing sugar-free gum or lozenges to stimulate saliva production.
About the Author
Theresa Tamkins is a health writer and editor who has worked for BuzzFeed News, Health.com, Reuters Health and other publications.