During a CNN town hall last week, a member of the audience asked Gary Johnson, the Libertarian candidate for president, about heroin legalization. Although the former New Mexico governor correctly pointed out that prohibition makes heroin use more dangerous, he disclaimed any interest in repealing it, saying his legalization agenda is limited to marijuana. He thereby undercut the utilitarian case against drug prohibition and missed an opportunity to make a moral case for individual freedom.

The Libertarian Party's platform states that "we favor the repeal of all laws creating 'crimes' without victims, such as the use of drugs for medicinal or recreational purposes." Johnson therefore was deviating from the party line when he declared that "we are not espousing the legalization of any drugs outside of marijuana." That was the easy way out, since most Americans recognize that marijuana is less hazardous than alcohol and think it should be legal. But what is the point of a Libertarian presidential campaign if it does not encourage voters to think about public policy issues in a more consistent and principled way?

With regard to alcohol and marijuana, Johnson said, "When it comes to choices in your own life, you should be able to make those choices as long as you're not doing harm to others." But he declined to extend that tolerance to other drugs, which makes no sense from a libertarian perspective. Either using force to protect people from their own risky decisions is legitimate, or it is not. If it is not, the specific nature of the decisions—whether they concern drugs, say, rather than food, sex or gambling, or heroin rather than alcohol or marijuana—should not matter. A government that respects individual freedom only insofar as it pertains to familiar or safe activities does not really respect individual freedom. Johnson should have said that any kind of drug prohibition violates the principle that each individual is sovereign over his own body and mind.

In addition to favoring political pragmatism over principle, Johnson's answer obscured the ways in which prohibition aggravates the problems it is aimed at solving. He alluded to some of those side effects but did not clearly connect them to the question he was asked, and he shied away from the logical conclusion that the problems caused by prohibition can be eliminated only by eliminating prohibition.

The heroin question came from Maureen Morella, a New Jersey woman whose 16-year-old son, Jesse, suffered brain damage after snorting heroin with his friends in 2004. "He became very sick and vomited," she explained. "He aspirated and was left with brain damage so severe that now, 12 years later, he remains in a wheelchair with no ability to eat or speak, and he is fed through a tube in his stomach."

One important point to make about this incident is that something very similar could have happened with alcohol. People who drink too much and pass out on their backs can (and periodically do) choke on their vomit, which can result in death or permanent disability. The possibility of such outcomes is not usually considered an argument for bringing back alcohol prohibition, possibly because prohibition did not protect drinkers from fatal or disabling accidents. If anything, it made such incidents more likely by encouraging quick consumption of potent beverages on the sly.

Likewise with heroin. The New York Times reports that Jesse Morella "took what doctors believe was a 'hot load,' a batch of heroin mixed with other chemicals, producing a toxic reaction." That sort of adulteration is a familiar feature of the black market created by prohibition, which makes drug quality and potency inconsistent, sometimes with lethal results.

Johnson tried to make that point, arguing that "prohibition, really, is what your son succumbed to," but muddied the argument by talking specifically about addicts. "You're a heroin addict," he said. "Your supplier has now been arrested and put into jail….Now comes a new supplier of heroin, and the new supply of heroin, visually you're taking the same dose that you've taken before, but it's of a different quality and a different quantity, and it ends up killing you." Then Johnson launched into a discussion of heroin prescriptions and needle exchanges for addicts, reinforcing the impression that his answer did not apply to novice users like Jesse Morella.

"I'm not talking about people that are addicted," Maureen Morella objected. But the unpredictable content of black-market heroin makes it more hazardous for anyone who injects, smokes, or snorts it, including first-time and occasional users.

Prohibition makes heroin use more dangerous in several other ways that cause a lot of damage even if they did not play a role in this particular case. Criminalization discourages education about especially risky practices such as mixing heroin with other drugs—a factor in the vast majority of heroin-related deaths. When a heroin user needs medical attention, his companions may be reluctant to call 911 because they could face criminal charges for possession or distribution. Prohibition fosters the spread of blood-borne diseases such as AIDS and hepatitis by encouraging injection (the most efficient way to consume an artificially expensive drug) and impeding access to clean hypodermic needles. And even users who avoid dangerous drug mixtures and eschew injection face the risk of arrest, along with the risk of black-market violence.

In short, prohibition hurts the people it does not deter for the sake of those it does. It manifestly did not prevent what happened to Jesse Morella and in fact seems to have contributed to his injuries, which makes him a poor exhibit in the case for keeping heroin illegal. Johnson suggested as much but muddled his message by agreeing that the government should "keep the drugs illegal" while trying to ameliorate the damage caused by that policy through "harm reduction" programs such as needle exchange, safe injection rooms, and the provision of pharmaceutical-quality heroin to addicts.

While there is nothing wrong with harm reduction, there is an important moral distinction between the harm that drug users do to themselves and the harm that the state inflicts on them in the name of deterring others. That policy is worse than mere paternalism, since the people it hurts are different from the people it supposedly helps (who may or may not actually exist). I do not expect a Republican or a Democrat running for president to recognize the injustice of that policy. But a Libertarian should.

This article originally appeared at Forbes.com.