What is, overall, safer for your body: alcohol or marijuana?
The health impact of any drug depends on how it's used, who's using it, how much is used, and under what circumstances. Marijuana and alcohol are no exception, so comparing them directly is difficult — each possesses the potential for unique risks or benefits.
For instance, alcohol is known to contribute to liver disease, but studies also suggest that a glass of red wine a couple of times per week can help keep your heart healthy. Similarly, marijuana is known to cause short-term cognitive impairment (trouble thinking clearly), but for certain medical conditions, such as glaucoma, marijuana possesses therapeutic properties.
On the other hand, people using marijuana or alcohol with the intention of getting high or drunk are more likely to experience the better-known negative consequences of these drugs. Alcohol use may contribute to memory loss; impaired judgment; academic, relationship, or work problems; along with long-term impacts like liver disease, heart disease, peptic ulcers, and physical dependency. Marijuana use may contribute to problems thinking clearly, memory loss, cognative deficiencies, dependency, and (if smoked) smoking-related health problems, such as throat and lung cancer and respiratory problems. Furthermore, both alcohol and marijuana are thought to contribute to mental health problems, such as depression and anxiety.
One major difference worth noting is that it isn't possible to ingest a fatal dose of THC (the active compound in marijuana) from smoking or eating marijuana. On the flip side, lots of alcohol in a brief period of time can lead to alcohol poisoning, an emergency medical condition that can be fatal.
Aside from direct health effects, getting behind the wheel while drunk or high poses significant risks to you and others. Marijuana and alcohol have both been shown to significantly reduce reaction ability, focus, and motor coordination; skills that are essential to safely operate a motor vehicle. In fact, many activities, such as playing a sport or even riding the subway, become dangerous when drunk or high. In general, being drunk or high compromises sound judgment and may lead to unhealthy or risky decisions that you wouldn't make when sober (like unprotected sex).
The list of facts and comparisons goes on and on, so check out the other Q&As in Alice's Alcohol, Nicotine, and Other Drugs archive for more information. In short, used moderately (in the case of alcohol) and/or under the direction of a physician (in places where medical marijuana is legal), these substances may have positive health effects. But as is often the case, heavy use can lead to heavy health risks.