I think it’s very likely that under drug legalization, the mix of drugs people are taking could change, but total drug-related harms wouldn’t increase. It’s far more likely these harms would decrease substantially. There are countervailing forces here, so we have to be clear about what these forces are and somehow estimate their magnitudes. (I’ll give an incomplete list what those forces are below, but this won’t be a thorough, rigorous cost-benefit analysis.)
Consider the forces that would decrease drug-related harms:
1) Some currently illegal drugs are extremely non-toxic, contrary to popular belief. Their legal status, while not exactly making them “unavailable”, does raise their relative cost. Legalization, at least for these drugs, would shift consumption toward these and away from the current legal drugs (alcohol, tobacco, and pharmaceuticals), which are more toxic and more prone to addiction problems.
2) Related to 1), but the same point for drugs that are potentially harmful. Someone might satisfy their need to get intoxicated on a low dose of cocaine or heroin, while needing a very much larger (and overall more harmful) dose of alcohol to achieve the same satisfaction. Don’t scoff at the notion that substituting from alcohol to heroin or from nicotine to methamphetamine could have net positive health and social effects. These drugs are not harmful if properly administered, and some of them (particularly opioids) don’t cause cumulative organ damage.
3) Currently illegal drugs are made *far more* dangerous by prohibition than they would be in a legal market. Drug interdiction causes the dosage to fluctuate wildly, which is the cause of a lot of overdoses. Often the active ingredient is a completely different chemical than the user realizes (fentanyl sold as heroin, or experimental designer drugs sold as MDMA). Impurities and adulterants are hard to control in a black market. Dealers have a preference for very concentrated doses of the drugs, because they are easier to transfer and smuggle, so these higher-concentration versions of drugs dominate the market.
4) It’s easier to advise people against self-harming behaviors in a legal market. Warning labels against specific harmful behaviors can be put on the packaging and purchasing the substance puts the buyer in front of a pharmacist who can advise them. Someone who buys two drugs that interact can be told “Don’t mix those.” (Considering that most drug poisoningsare multi-substance interactions, this would be a very important behavioral tweak.) Relatedly, it becomes easier to usher someone into drug rehab if their habit doesn’t make them a criminal. More people are likely to seek necessary help if doing so doesn’t risk legal sanctions.
5) The legal sanctions are themselves harmful to people’s lives, and these would obviously disappear under full legalization. These include decades in prison, low-level harassment of millions of innocent people, and the occasional beating or shooting of a drug suspect. We’d do away with this class of social harms entirely.
6) Black market violence is a huge driver of our overall crime rates. Estimates vary, but by all accounts are big. Read “Drug War Crimes” by Jeffrey Miron for the full story, but something like ¼ to ¾ of our murders can be blamed on the drug war, and that’s not even the entire class of black-market social costs.
Now consider the force that would lead to an increase in drug-related harms:
1) Someone who otherwise wouldn’t have tried cocaine (or some other illegal addictive substance) now tries it and develops a self-harming habit. This is the opposite of 2) above. These people require a large, harmful quantity of illegal drugs to become satisfied. In other words, they are willing to undergo the self-harm for cocaine (and heroin, etc.), but not for the effects of alcohol or something else that is relatively inexpensive under our current regime.
For drug legalization to be a bad idea, the one harm-increasing force would have to dominate all the harm-reducing forces. I find this implausible. Don’t interpret this as a “This list is longer than that list, therefore…” argument. It’s possible in principle for the one harm to overwhelm all the benefits, even though it’s one item. I could arbitrarily express it as four items with some clever manipulation, or compress my list of benefits. We have to weight these things somehow, not simply count them. Still, I think drug legalization comes out ahead. The one “harm-increasing” force is almost surely small. There just aren’t that many people who 1) are willing to endure enormous amounts of self-harm and 2) are deterred significantly by weak legal sanctions. (Here is a more rigorous treatment of the problem, stated in terms of demand curves and total costs.)
Prohibition probably causes people to substitute some drugs in place of others, but it doesn’t alter their underlying willingness to undergo self-harm. If you look at evidence from alcohol and tobacco, it appears that the self-harm dominates the dollar price. No doubt this result generalizes to other addictive substances. The goal of drug policy should be to reduce the social harms of drugs, not to reduce drug use *per se*. This goal can be achieved by allowing people to achieve their desired state of intoxication in the least harmful way possible. If we had a wider array of intoxicants (some of them almost entirely non-toxic) available to us, this would be a lot easier.
The fear is that light dabbling in these other kinds of drugs will inevitably lead to full-fledged addiction, with all the overdoses and other health problems. But this “try it once and you’re hooked” story is a myth. It doesn’t describe the vast majority of illegal drug use. Normal, well-adjusted people have nothing to worry about. It’s the impulsive people with self-control problems that we need to worry about, and these people are mostly already hurting themselves. Drugs don’t dominate the will and turn the former type of person into the latter. There is just an enormous amount to gain here and very little to lose. Legalizing drugs is the lowest of the low-hanging fruit. It's long past due that we pluck it already.