Dec. 11, 2012 -- On Monday, Colorado Gov. John Hickenlooper signed an order legalizing recreational marijuana use for adults.
Last week, Washington was the first state to OK adult recreational use of the drug. Several other states are considering similar laws.
WebMD turned to two experts, recently published studies, and the National Institute on Drug Abuse to draw up a scorecard of possible major health effects.
Health Effects of Marijuana: Lungs
"Putting smoke in your lungs is not good for the lungs," says Roland Lamarine, HSD, professor of public health at California State University, Chico. He reviewed published studies on the health effects of marijuana earlier this year for the Journal of Drug Education.
Smoking marijuana produces a nearly threefold increase of inhaled tar compared with tobacco, according to some studies. Other research suggests that marijuana smokers, compared to cigarette smokers, inhale more deeply and hold their breath longer.
"There are still questions that aren't answered about lung damage," Lamarine says. For cigarette smokers who also smoke marijuana, there may be an additive effect, he says.
Combining the two appears to be a trend, he says. "Some of the [college] kids tell me they buy cigars and put in some marijuana, so there is both marijuana and tobacco," Lamarine says.
Marijuana smoke contains cancer-causing substances, according to the National Institute on Drug Abuse. Some research shows that marijuana smoke has up to 70% more cancer-causing substances than tobacco smoke, it says.
"Nobody is advocating that routinely inhaling carcinogenic smoke is healthy," says Paul Armentano, the deputy director of NORML (National Organization for the Reform of Marijuana Laws).
However, he says, many marijuana users these days have turned to alternate delivery methods, such as oral, tinctures, and vapor forms. In research, he says, the vaporized forms have fewer adverse chemicals than the inhaled form.
Those who keep marijuana use light do not appear to lose lung function, according to a 2012 study in the Journal of the American Medical Association.
Researchers compared tobacco and marijuana users. Tobacco use was associated with lower lung function, and the function got worse as smoking levels increased.
For the study, low levels of lifetime exposure to marijuana, defined as one joint a day for seven years, did not show evidence of adverse effects on lung function. The study does not confirm these findings with heavy users.
Health Effects of Marijuana: Brain Impairment
Long-term, heavy use of marijuana can lead to impaired thinking skills and memory problems, Lamarine says, citing published studies.
The impairments can be especially hazardous when trying to do everyday activities such as driving.
However, these impairments appear to be most significant 20 to 40 minutes after using marijuana, then decline after an hour or so, Armentano says.
Individual reactions vary, of course. To be safe, he says, ''I think a three-hour window [between marijuana use and driving] is appropriate for [after] inhalation."
Lamarine says the effects may last longer than that. He says more research needs to be done for a definitive answer.
"I'd say three hours [between marijuana use and driving] is probably pushing it." However, he says, waiting three hours would be better than waiting less time, as he suspects many users now allow.
In a recent study, Yale University researchers looked at the effects of both marijuana and alcohol on driving. Both impair driving-related skills. But they found the impairment effects of marijuana, compared to those of alcohol, vary more among people.
That is thought to be due to differences in tolerance, smoking technique, and the potency of the marijuana.
While studies are conflicting about whether marijuana use alone leads to more accidents, combining it with alcohol definitely raises crash risks, experts say.
Health Effects of Marijuana: Mental Health
However, Lamarine suspects the relationship may be reversed. "What I suspect is, people who are going to develop full-blown schizophrenia must feel bad beforehand," he says. They may be attracted to marijuana as a mood-altering drug to self-medicate.
If this link were cause and effect, Armentano says, the rates of schizophrenia would have climbed, as marijuana use has increased.
Marijuana use can be linked with anxiety, Lamarine and Armentano agree.
Several published studies, Lamarine says, have found a link between marijuana use and anxiety, but not necessarily full-blown anxiety disorders.
Studies have also found a link between regular marijuana use and panic attacks.
Some marijuana users do have panic attacks, especially if they are new to the drug, Armentano says.
As for the anxiety-marijuana link, he says, it could be that some anxious people try marijuana to self-medicate.
Those who have known psychotic problems should proceed with caution when thinking about marijuana use, Armentano says.