Robert Gable is an emeritus professor of psychology at Claremont Graduate University.

Twenty years ago, when my son was a teenager, his favorite recreational drug was ecstasy, or MDMA. He claimed that ecstasy was “as safe as alcohol,” and that there was nothing to worry about. Of course, I did worry.

My worry eventually led to a decade-long research project that compared the relative risk of 20 different recreational substances. Here’s the way scientists usually make comparisons of the risk (acute lethality) of various drugs:

No drug is good for teenagers. But when it comes to the chances of immediate death by chemical toxicity, marijuana is about a hundred times safer than alcohol.

First, determine the “effective” dose of the drug. In the case of alcohol, for example, two 12-ounce beers or two 1.5-ounce shots of 80-proof vodka will give a normally healthy teenager a substantial buzz. The beer or the vodka each contain about 33 grams of ethyl alcohol.

Second, determine the “lethal” dose of the drug. Again, in the case of alcohol, hospital records show that people who die from an alcohol overdose have usually ingested about 330 grams of alcohol — equivalent to 20 shots of vodka.

Third, divide the lethal dose by the effective dose. This gives the “safety margin” of the drug. For alcohol, the safety margin is 10 (330 divided by 33 equals 10). In other words, it takes 10 times as much alcohol to kill you as it does to give you a buzz. Note: All such calculations are very rough estimates, and severe toxic reactions can occur at much lower doses depending on the health of the individual.

Based on my research, the safety margin of recreational substances as normally used is: 6 for heroin; 10 for alcohol; 15 for cocaine; 16 for MDMA; 20 for codeine; and 1,000 for LSD or marijuana. Cigarettes have little immediate risk of death because most of the very lethal nicotine is destroyed in the smoke. The long-term risk of addiction and cancer from cigarettes is well known.

No drug is good for teenagers. But when it comes to the chances of immediate death by chemical toxicity, marijuana is about a hundred times safer than alcohol or cocaine.