During Prohibition, drinkers never knew what they would get when they set out to slake their thirst. Bootleggers often sold products adulterated with industrial alcohol and other toxins. Some 10,000 people were fatally poisoned before America gave up this grand experiment in suppressing vice.

So it was a tragedy but not a total surprise when three deaths were reported in Illinois from synthetic marijuana laced with an ingredient (possibly rat poison) that caused severe bleeding. Nationally, in 2015, says the Drug Policy Alliance, "poison control centers received just under 10,000 calls reporting adverse reactions to synthetic cannabinoids, and emergency rooms received tens of thousands of patients."

People consume synthetic cannabis for the same reason people once consumed bathtub gin: Their drug of choice is illegal. Criminal organizations that cater to forbidden demands don't always make a fetish of quality control. After Prohibition was repealed, though, tipplers could buy from legal, regulated suppliers. They no longer had to worry about ingesting sudden death.

In nine states and the District of Columbia, pot users now enjoy the same protection. Recreational marijuana is allowed and subject to government regulation and the discipline of the market—ensuring purity through accountability. But in most places, Americans who want to get high have to take their chances with unsanctioned dealers who may be sorely lacking in moral scruples.

The bigger toll from modern drug prohibition, however, comes among opioid users. By making criminals of many people who are dependent on prescription painkillers such as oxycodone and hydrocodone, the law exiles them to the black market. There, consumers may find legitimate FDA-approved medicines, but they may also buy counterfeit versions or heroin—which often carry far greater hazards.

The most urgent danger comes from fentanyl, an opioid at least 30 times more powerful than heroin that illicit producers often mix with other opioids. It plays a rapidly growing role in the epidemic of drug overdose deaths.

The number of deaths caused by fentanyl and other synthetic opioids, says the National Center for Health Statistics, increased by 88 percent per year from 2013 through 2016. In 2016, these drugs killed more than 19,000 people.

Why would traffickers cut a dangerous drug (heroin, oxycodone) with an even more dangerous one? Fentanyl's low cost and high potency allow sellers to make more money. The iron law of prohibition stipulates that banning a substance encourages more powerful alternatives because they are more compact and thus easier to hide (boxes of pills versus bales of marijuana). The side effect is to greatly compound the dangers of drug use.

As if its role in opioids weren't bad enough, fentanyl has shown up in cocaine. Law enforcement agencies in Connecticut and Massachusetts report a surge in this particular mixture, which is especially dangerous because cocaine users usually lack a tolerance for opioids.

Fentanyl was just the beginning. The latest additive is carfentanil, a compound 100 times more powerful than fentanyl that is used to tranquilize elephants. It's shown up in a street drug known as "gray death," which sells for much less than pharmaceutical opioids. Its advent is likely to boost the casualty count.

These side effects are an inevitable result of treating a vice, or a medical condition, as something to be punished. The simplest way to curb the epidemic would be to make it possible for those addicted to opioids to obtain and use them legally. Pharmacists don't mix up cocktails with sedatives meant for animals weighing 6 tons.

Short of some form of legalization, useful steps could be taken. Drug testing kits can detect the presence of fentanyl and other contaminants—but in many places, including Illinois, they are classified as illegal drug paraphernalia. The District of Columbia recently decided to grant an exemption letting syringe exchange programs screen drugs for the people they serve.

In some states, syringe exchange programs don't do that because there aren't any. Twenty-two states criminalize the mere possession of hypodermic needles.

It would help to have facilities where opioid users could inject drugs under the supervision of medical professionals who could intervene to reverse overdoses—not to mention offer counseling and treatment referrals.

In 2016 alone, more Americans died of overdoses than were killed in the Vietnam War. Drug prohibition is justified as a vital protection against the ravages of abuse and addiction. But our graveyards are filling up with people it was supposed to save.