According to virtually every scientific review, including a 2016 World Health Organization report and a 2017 National Academy of Sciences study, marijuana is addictive and harmful, despite rhetoric from the marijuana industry. The chances of becoming dependent on marijuana can be up to 50% for some users, and regular use is indisputably dangerous to the adolescent brain, in some cases linked to permanent reductions in IQ.
Unlike cigarettes, marijuana also intoxicates, sometimes with tragic results. The percentage of traffic deaths related to marijuana more than doubled in Washington State the year retail marijuana sales were allowed, and Colorado has seen similar increases in pot-related accidents.
A "War on Drugs" approach to marijuana is ineffective and counterproductive. It simply doesn't make sense to spend limited resources going after individual non-violent drug users. But the other extreme - legalization - is also a serious mistake. Commercializing marijuana will create the next "Big Tobacco" industry of our time. A new industry of lobbyists and special interests intends to put profits and special interests over public health and safety.
Alcohol is not legal because it is safe. It is legal because most of Western civilization has used the drug for thousands of years. By contrast, marijuana has always been used by a small minority of the population. Drug laws keep rates of use down, thereby lowering the negative consequences for communities. Alcohol and tobacco, two legally available drugs, provide a good example, since Americans use them far more frequently than illegal drugs.
Alcohol is in and out of your system within 24 hours; marijuana's effects last much longer. THC, the psychoactive substance in marijuana is fat soluble, meaning it stays in the system much longer. Research has found that marijuana abusers self-report far worse outcomes than alcohol users, including more problems at home, work, or school and more mental health problems.
Evidence also finds that people often use alcohol, marijuana, and tobacco together. Rarely do users substitute alcohol for marijuana. In fact, rates of alcohol sales continue to rise in states that have legalized marijuana. Do most people who use marijuana go on to other drugs? No, but do most people who use other drugs start with marijuana? Most definitely.
A wide array of research has confirmed links between marijuana use and other drugs. While it is true most marijuana users won't go on to use other drugs, research demonstrates that 99% of those addicted to other drugs started with alcohol and marijuana. Marijuana users are also three times more likely than non-users to become addicted to heroin, and a 2017 National Academy of Sciences report found a statistical association between marijuana use and the development of substance dependence for other drugs like opioids and heroin.
It's also important to keep in mind that marijuana on its own is harmful. The average potency of marijuana has skyrocketed since the 1970's and research demonstrates it is associated with substance use disorders, drugged driving crashes, lower IQ, and other negative consequences. Today, more young people are voluntarily seeking treatment for marijuana addiction than for all other drugs combined, including alcohol.