Marijuana & Diving

Marijuana (cannabis) is one of the most commonly used recreational drugs in the world. In 2015 a Pew Research Center survey found that almost half of Americans claim to have used marijuana at some point in their lives.1 According to the 2014 National Survey on Drug Use and Health (NSDUH) prepared for the Substance Abuse and Mental Health Services Administration (SAMHSA), some 22 million Americans ages 12 and older smoked marijuana in a single month that year.2

Marijuana, like heroin, is a Schedule I controlled substance in the U.S., a classification reserved for what the government considers to be the most dangerous drugs.3 The Drug Enforcement Administration (DEA), however, recently revealed that it plans to decide this summer whether marijuana should be removed from that list.4 Four states — Colorado, Washington, Oregon and Alaska — and the District of Columbia have already passed measures to legalize marijuana for recreational use.

U.S. Surgeon General Vivek Murthy, M.D., told CBS This Morning in February 2015 about his stance on marijuana legalization: “We have some preliminary data showing that for certain medical conditions and symptoms, marijuana can be helpful.” In a statement issued by the Department of Health and Human Services, he added, “Marijuana policy — and all public health policies — should be driven by science.”5

Marijuana for medical use is currently legal in 25 states, the District of Columbia and Guam. It is prescribed to treat a variety of severe and debilitating ailments including multiple sclerosis, muscular dystrophy, terminal cancer, AIDS, Crohn’s disease, Parkinson’s disease, inflammatory bowel disease, severe or chronic pain and glaucoma,6 and it is currently being studied for use in treating post-traumatic stress disorder (PTSD).7 In Canada, Prime Minster Justin Trudeau has asked the Minister of Justice and Attorney-General Jody Wilson-Raybould to “create a federal-provincial-territorial process that will lead to the legalization and regulation of marijuana.”8

As marijuana legalization proceeds in the U.S. and Canada, the number of people, including divers, using marijuana may increase.

Scuba divers are already smoking marijuana in significant numbers and without any guidelines about smoking and diving practices. In 2011 Marguerite St Leger Dowse and colleagues published a survey of recreational divers in the U.K. who used illicit drugs, including marijuana. Of the 479 divers who responded, 105 (22 percent) reported using illicit drugs since they first learned to dive, and of those 105, 99 (94 percent) had used marijuana.9

With this data in mind, there are questions about marijuana and diving that warrant answers. To date, marijuana research in the United States has been very difficult because of marijuana’s classification as a Schedule I drug, Those restrictions may change if the government downgrades marijuana to a lower category. In the meantime, we ask the experts.

What are the most immediate concerns with regard to marijuana use and diving?

Paul Auerbach: When considering the effects of marijuana on divers, a number of issues come to mind. There are acute and chronic effects that may influence a diver’s mental state, judgment and physical reflexes as well as perhaps relevant cardiovascular, neurological and other physiologies. It is dictum that anything that might impair a diver underwater is detrimental and forbidden, unless there is a benefit that supersedes the negative effects. We know virtually nothing about the effects of marijuana use in divers, but there is scientific literature on the use of marijuana above water from which we can reasonably extrapolate. Common sense dictates that the onus is on the science community to prove that marijuana use in any form or quantity in divers is safe, rather than to take a position that it should be considered safe until proved otherwise.

Tom Neuman: In a laboratory setting, marijuana clearly reduces a number of motor and cognitive skills and thus, if one can extrapolate from the laboratory setting to real-life situations, the use of marijuana prior to diving (i.e., diving while “high”) would be problematic.

Are people who are ill enough to warrant medical marijuana fit enough for diving?

Steven Gillon: It depends on why they are taking marijuana and the degree to which they have a specific condition. I prescribe medical marijuana for patients with severe illnesses such as Crohn’s disease, irritable bowel syndrome and intractable pain. There are many other indications as well. Some of these illnesses are controlled, while others are not, and of course those situations preclude diving. I would also strictly advise against marijuana use and diving for anyone with cardiovascular or pulmonary disease or compromised vestibular function until they are appropriately assessed for fitness to dive.

Does marijuana have an adverse effect on a diver’s lung function, and how does marijuana affect a diver’s cardiovascular system?

Ernest Campbell: It is good to answer these two questions together, as lung and cardiovascular functions are closely related. Marijuana smoke, in addition to containing THC and cannabinoids, also has large quantities of tar, foreign bodies and carcinogens (50 percent more than tobacco smoke). The acute effects of smoking marijuana on a diver’s lungs include an increase in size of the terminal bronchi, increase in mucus production, cough, inflammation and loss of function of the ciliated bronchial cells (which clean dust and debris and perform an immune function, reducing infection). This process increases the risk of mucus plugs, air trapping and gas embolism. Chronic smoking can lead to chronic obstructive pulmonary disease, with the concomitant risks of air trapping and gas embolism.

Cardiovascular changes that occur with episodic smoking of marijuana include an increase in the heart rate by 30-50 beats per minute, increase in blood pressure and gas changes that can include decreased oxygen and increased carbon monoxide (CO) and carbon dioxide (CO2) with an elevated risk for heart attack and syncope. Small gas changes at the surface become exaggerated with the effects of depth/pressure and may lead to CO poisoning at depth and hypoxia upon ascent.

To what extent will marijuana exacerbate nitrogen narcosis?

Richard Moon: It is well established that nitrogen under pressure causes cognitive changes and can alter reaction time, judgment and performance in a number of areas. The effects of nitrogen narcosis begin to be most apparent at depths between 80 to 100 feet of seawater, and they become progressively more marked as depth increases. We know that there is interaction between nitrogen and other gasses that can cause cognitive deficits, such as elevated blood CO2 levels. High levels of oxygen, such as might be experienced with enriched oxygen mixtures such as nitrox, may exacerbate nitrogen narcosis. Marijuana use, which is known to have adverse effects on cognition and performance, is likely to exacerbate nitrogen narcosis. The effects of marijuana on divers need to be investigated before any definite statements can be made regarding safety. In the meantime, extreme caution is recommended.

Neuman: There are no data that currently exist to suggest whether or not the use of marijuana would exacerbate or possibly ameliorate the effects of nitrogen narcosis. We do know that alcohol and marijuana act additively, and thus it would not be a surprise if the effects of marijuana exacerbated the effects of nitrogen narcosis. In a laboratory setting it would be relatively simple to examine this question, however, validating the results of laboratory testing in an actual diving environment would require separate testing. The first step in further evaluating this question might be conducting chamber dives to depths at which we already know nitrogen narcosis produces measurable effects.

How long after using marijuana should someone wait before diving?

Gillon: Like alcohol, marijuana should be undetectable in a person’s system before he dives. When marijuana is inhaled at sea level, the THC metabolites have an elimination half-life of at least 20 hours and possibly up to three days. Some are stored in body fat, however, and have an elimination half-life of 10 to 13 days. Most researchers agree that urine tests for marijuana can detect the presence of the drug in the body for up to 13 days. But for some individuals it may remain even longer. There is anecdotal evidence that the length of time that marijuana remains in the body is affected by how often the person smokes, how much he smokes and how long he has been smoking. Regular smokers have reported positive drug test results after 45 days since last use, and heavy smokers have reported positive tests up to 15 weeks. So without human trials specific to marijuana use and diving, we can only guess at safety parameters, and they would have to be extremely conservative.

Is there currently any way to establish a correlation between the use of marijuana and diving accidents?

Neuman: If one were to use the data from motor-vehicle accidents as a surrogate for dive accidents, the results are not as clear-cut as the laboratory evidence might suggest. Adverse effects of marijuana have not been conclusively demonstrated in experiments involving simulated driving. Furthermore, in epidemiologic studies of motor-vehicle accidents, marijuana has not been shown to be a major problem. Unfortunately, all of the existing studies have major methodologic problems (i.e., are flawed). [Author’s note: Research funded by drug companies may be influenced by the potential profits associated with getting drugs to market as quickly as possible. Maida Taylor, M.D., MPH, who has worked in drug development, notes, “There is a problem in the research that has been done on marijuana by the pharmaceutical companies, even in the most controlled circumstances, because of the strong sociopolitical agenda.”]

Perhaps most important, no way currently exists to determine (with laboratory testing) whether an individual is under the influence of marijuana or has merely been exposed to it sometime in the past hours to days. This problem is related to the way marijuana is metabolized and eliminated from the body; it is not merely a technical issue of detection. Furthermore, the active agent in marijuana is usually not detectable in the bloodstream after two hours from the time of inhalation. Thus, by the time a person arrives at a hyperbaric chamber the likelihood that useful information could be obtained by any currently available blood test is extremely small.

Assuming this technical problem is resolved, there are still extremely large problems that will need to be overcome before any causative relationships between marijuana use and dive accidents can be established. The single greatest issue will be the ability to find an adequate control group for comparison. Given the relatively small number of diving accidents, the difficulty in establishing a meaningful control group and the technical difficulties in establishing a relationship between laboratory testing and impaired performance in a real-life setting, it may be quite some time before we can clearly delineate the role of marijuana in diving accidents. Until then, prudence would suggest extreme caution in this arena.

Meet the Experts

Paul S. Auerbach, M.D., MS, FACEP, FAWM, FAAEM, is the Redlich Family Professor of Emergency Medicine at Stanford University School of Medicine and medical director of Strategic Projects at Stanford Health Care. He is the world’s leading authority on wilderness medicine and one of the world’s leading authorities on emergency medicine. He has received numerous science and diving awards.

Ernest S. Campbell, M.D., FACS, a retired surgeon, is a Fellow of the American College of Surgeons and a member of the American College of Physician Executives, among other affiliations. At Brookwood Medical Center in Birmingham, Ala., he served as chairman of the surgery department and president of the medical staff. He is a PADI Rescue Diver and has a U.S. Coast Guard captain’s license.

Steven D. Gillon, DO, is a gastroenterologist and owner of the Digestive Health Center of Englewood in Englewood, N.J. He has been practicing gastroenterology and internal medicine for 30 years, and he prescribes marijuana for selected medical cases that meet the legal requirements.

Richard E. Moon, M.D., CM, M.Sc., FRCP(C), FACP, FCCP, is a professor of anesthesiology and medicine at Duke University in Durham, N.C. He is also the medical director for the Duke Center for Hyperbaric Medicine and Environmental Physiology. A world-renowned researcher in anesthesiology, pulmonary, undersea, environmental and hyperbaric medicine, he has authored more than 100 peer-reviewed publications.

Tom S. Neuman, M.D., FACP, FACPM, an emergency medicine physician and pulmonologist in La Jolla, Calif., is well known for his work in diving and hyperbaric medicine. He has published numerous books and scientific papers on diving physiology and is the co-editor of the Physiology and Medicine of Diving. Neuman is a recipient of the DAN/Rolex Diver of the Year Award for his contributions to dive safety.


1. Motel S. Six Facts about Marijuana. Pew Research Center. Published April 4, 2015. Accessed July 1, 2016.

2. Hedden SL, Kennet J, Lipari R, Medley G, Tice P. Behavioral Health Trends in the United States: Results from the 2014 National Survey on Drug Use and Health (HHS Publication No. SMA 15-4927, NSDUH Series H-50). Rockville, Md.: Substance Abuse and Mental Health Services Administration, Center for Behavioral Health Statistics and Quality, Substance Abuse and Mental Health Services Administration; 2015. Available at

3. Controlled substance schedules. U.S. Department of Justice Office of Diversion Control. Published May 2016. Accessed July 1, 2016.

4. Ingraham C. The DEA will decide whether to change course on marijuana by July. The Washington Post. April 6, 2016. Accessed July 1, 2016.

5. Ferner M. U.S. Surgeon General Vivek Murthy says marijuana ‘can be helpful’ for some medical conditions. The Huffington Post. February 4, 2015. Accessed July 1, 2016.

6. States Permitting Use of Medical Marijuana and approved indications. Pharmacy Purchasing and Products. Published July 31, 2014. Accessed July 1, 2016.

7. DEA Approves First-Ever Trial of Medical Marijuana for PTSD in Veterans [news release]. Multidisciplinary Association for Psychedelic Studies; April 21, 2016. Accessed July 1, 2016.

8. Plucinska J. Justin Trudeau takes a first step toward marijuana legalization in Canada. Time. November 16, 2015. Accessed July 1, 2016.

9. Dowse MS, Shaw S, Cridge C, Smerdon G. The use of drugs by UK recreational divers: illicit drugs. Diving Hyperb Med. 2011 Mar; 41(1):9-15.

© Alert Diver — Q3 Summer 2016