The Dilemma: You’ve recently been diagnosed with ADHD and are wondering whether or not your doctor has prescribed you crank to treat your hyperactivity. Because while you’re not an MD or anything, that seems like a bad idea.

People You Can Impress: This is the rare bit of knowledge you can use to impress both chemists and drug addicts.

The Quick Trick: If you’re taking diet pills, that’s amphetamine. If you’re smoking crystal, that’s methamphetamine (and also not so good for you).

The Explanation: The difference here, we regret to report, involves some polysyllabic chemistry, but on the up side, the periodic table need not be mentioned. Both drugs are stimulants of the central nervous system, just like MDMA, commonly known as Ecstasy. But amphetamine is known technically as methylated phenylethylamine, while methamphetamine is known as double methylated phenylethylamine. The second methylation (to make up a word) changes the compound’s interaction with the body.

Regular amphetamine can be plenty bad for you. Speed is found in everything from the ADHD drug Adderall to diet pills. Many nations, including the U.S., also sometimes give amphetamines to members of the armed forces to increase alertness. But it comes with more than a couple problems: First, it’s addictive. Second, it can cause heart attacks. Third, it can cause “amphetamine psychosis,” which is very similar to schizophrenia except you have more energy. But such side effects are rarely a problem for those who take amphetamines as prescribed.

Methamphetamine, on the other hand, is widely considered too dangerous to be prescribed. Somewhat stronger than an amphetamine, meth was first synthesized by a Japanese scientist in 1919. Widely prescribed in America and abroad in the 1950s, meth was used to treat everything from alcoholism (ironic because, at best, it only caused a switch in addictions) to Parkinson’s (ironic because meth causes involuntary body tics). The production of meth, which involves mixing over-the-counter cold medication with hydriodic acid, wasn’t even illegal in much of the U.S. until 1986.

Long known as a drug abused mostly by truckers and bikers, meth only spread into the larger population in the 1980s. But by the year 2000, 4 percent of Americans polled acknowledged having used meth at least once. The allure of meth is that it’s very cheap and makes you very high—the drug gives you a feeling of ecstasy caused by dopamine flooding the central nervous syndrome. Unfortunately, this eventually leads to irreversible brain damage.

But that’s not all. Chronic abuse is associated with paranoia, hallucinations, strokes, and dementia. Also, it is exceptionally bad for your breath. And it’s no fun to get off the stuff : Withdrawal symptoms include seizures.

This post was excerpted from the mental_floss book What’s the Difference? For more columns like this, click here.