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The toxicology report has come in on Rudy Eugene, the perpetrator of a brutal face-eating attack on a Miami causeway that left a homeless man permanently disfigured. After weeks of breathless speculation about the drugs that could have caused this case of zombie cannibalism — namely so-called bath salts — the Miami-Dade medical examiner found no such drug in Eugene’s system.

The sole substance detected? Marijuana.

The Miami Herald reports:

The [medical examiner’s] office said it sought the help of an outside forensic toxicology lab, “which has confirmed the absence of ‘bath salts,’ synthetic marijuana and LSD.” The ME’s office said that “within the limits of current technology by both laboratories,” marijuana was the only drug found in Eugene’s system.

Eugene was shot dead by police on May 26, when he would not cease his gory assault. Widespread media reports afterward suggested that he could have been taking the synthetic stimulant drugs sold as bath salts, which have been previously associated with other cases of psychotic behavior.

Despite the fact that Eugene had no synthetic drugs in his system, it’s likely that his case will still be used for years as an example of what bath salts can make people do. If our history of wrong-headed beliefs about drugs is any indication, the association between bath salts and dangerous behavior will stick simply because it surfaced early. Since bath salts just arrived on the illegal market and remain untested and mysterious to many people, they will cling in ignorance to the horror stories about them.

In contrast, the same hysteria no longer applies to marijuana because America’s widespread personal experience with the drug — nearly 70% of people in some age groups have tried it — means they know it doesn’t trigger violent behavior in otherwise sane people.

That wasn’t the case in the early 20th century, however. In the 1930s, after the release of the hyperbolic anti-marijuana film Reefer Madness, people took its claims — that smoking weed leads to killing, suicide, rape and general insanity — seriously, because most had no idea what marijuana was. They didn’t have the frame of reference necessary to reject the idea that it could cause irrational violence or moral deviance.

PCP was another drug that was demonized early on, with many suggesting that it caused uniquely bizarre violent behavior and superstrength. The “classic” cases mainly turned out not to be related to the drug. Nonetheless, these very myths were used by the police to justify their excessive use of force in the 1991 beating of L.A. motorist Rodney King. He, too, turned out not to have been on the drug when he was assaulted. (Indeed, PCP actually weakens muscle strength, at least in studies of mice.)

Similarly, in the early 1980s, with rising popularity of crack cocaine, Americans believed the drug was capable of driving women into prostitution and turning men into casual killers. The data later showed that the effects of the drug were no different from those of powder cocaine, which could also be injected to cause a high of equal intensity. The only thing different about crack was that it could be smoked.

The wave of “crack-caused” crime in the ’80s and ’90s was almost completely due to turf battles over the market for the new stuff. That is, it wasn’t crack use itself, but the violence related to drug dealing that resulted in crime. Media attention and crack-related crime has since fallen off, but the rate of daily crack use by high school seniors has stayed the same since it was first measured in 1987. That rate is 0.1%, although the proportion of high-school seniors who reported using crack at least once in the last year fell from 4% to 1%.

We don’t yet know all the ways in which bath salts affect users, but the history of drug scare stories and their negative effects on policy should give us pause. Misconceptions about crack resulted in harsh laws with racist effects: 1 in 3 young African American males enters the criminal justice system, a statistic largely driven by drug charges — a rate far higher than for whites, despite the fact that whites and blacks use drugs at similar rates.

The Miami case represents the tragedy of untreated mental illness, not drug use alone. While mental illness can be exacerbated by marijuana and other drugs, such cases shouldn’t lead to criminal crackdowns. That’s not to say that people should be allowed to take drugs that haven’t been thoroughly tested on humans — but current policies didn’t keep marijuana away from a seriously mentally ill man either.

If we want to prevent such violence, we need to focus on its genuine causes, not sensational claims about what drugs “make” people do.

Maia Szalavitz is a health writer at TIME.com. Find her on Twitter at @maiasz. You can also continue the discussion on TIME Healthland’s Facebook page and on Twitter at @TIMEHealthland.