But some experts caution that the mere presence of THC in a person's blood tells us little about whether he was actually experiencing effects of marijuana use at the time of the test. "You can test positive for THC days after you've smoked cannabis," according to Mark Kleiman, an expert on drug use and drug policy at UCLA. But most chemical tests don't distinguish active forms of the THC molecule - which indicate that a person is likely experiencing the effects of the drug - from inactive forms, which are a non-psychoactive byproduct of marijuana use, and which can linger in a person's body for days or weeks.
Brown's toxicology report doesn't indicate such a distinction, according to the St. Louis Post Dispatch, but it nonetheless states that the THC detected in Brown's blood defines impairment. "As a matter of law this may be correct," Kleiman says - in other words, there may be a legally-defined threshold for determining marijuana impairment. But given the uncertainties inherent to THC testing, "this does not tell you for sure the guy was impaired," Kleiman says.
In an interview with the St. Louis Post-Dispatch, Dr. Michael Bostwick, an addiction specialist at the Mayo Clinic, agreed: "Numbers do not in my experience predict behavior or even completely explain it,” he said. The National Highway Traffic Safety Administration also notes the difficulties of interpreting blood THC levels: "It is difficult to establish a relationship between a person's THC blood or plasma concentration and performance impairing effects. Concentrations of parent drug and metabolite are very dependent on pattern of use as well as dose."
Some have suggested that the THC levels in Brown's blood may have been high enough to trigger hallucinations. But experts say this is highly unlikely. Kleiman notes that "hallucinations are rare" with cannabis use.
However, anxiety and paranoia are more common than hallucinations, especially at high doses. It's therefore possible that marijuana use was a factor in the events leading up to Brown's death.
"Marijuana does impair judgment and thinking," according to Jonathan Caulkins of Carnegie-Mellon University. "Anything that clouds judgment or interferes with communication ability can promote misunderstanding, and misunderstanding can lead to bad resolutions of conflict situations."
In other words, if you're high and find yourself in the midst of a tense verbal altercation with a cop, you may not be in the best state to defuse the situation.
"Overall," Caulkins says, "marijuana use is not associated with elevated levels of violence the way that alcohol or even cocaine are." Kleiman notes that in the U.S., heavy marijuana use has gone up sevenfold from 1992 to today. But you don't see a corresponding increase in levels of violent behavior over the same period.
In the end, the presence of THC in Michael Brown's system doesn't tell us much about the events leading to his death. It does, however, point to the need for better ways to test for marijuana intoxication, especially as several states consider legalizing marijuana for recreational use.