There are two reasons for this. The first is that the pharmacokinetics of marijuana—the way it is absorbed and excreted by the body—are different for smoking and eating it. The second is “set and setting,” the way in which people use it and the expectations they bring—specifically, that it’s much easier to control the dose when smoking.
“In a nutshell, eaten cannabis gets metabolized by the liver, so delta-9 THC becomes 11-hydroxy-THC, which passes the blood-brain barrier more rapidly and has more of a psychedelic effect than standard THC,” says Understanding Marijuanaauthor Mitch Earleywine, a professor of psychology at the State University of New York at Albany. “Smoked or vaporized cannabis bypasses the liver and doesn’t create the same 11-hydroxy-THC.”
Smoking marijuana gets THC into the body much faster and at higher concentrations, but it stays there much longer after eating. With smoking, as much as 50 to 60 percent of the THC in a joint can get into the blood plasma, and peak concentrations come in 5 to 10 minutes. It “very quickly crosses the blood-brain barrier,” explains Paul Armentano, deputy director of the National Organization for the Reform of Marijuana Laws. With orally administered cannabis, only 10 to 20 percent of the cannabinoids reach the blood plasma, and they do so 60 to 120 minutes later, says Dr. Mark A. Ware, an associate professor of family health at McGill University in Montreal.
Those who eat cannabis have to estimate what the dose they’ve taken will do, and the length of the high means that any unpleasant effects take longer to go away.
How long a drug stays in the body is usually measured—like radioactivity—by half-life, or how long it takes for half the drug to disappear from the blood. This is complicated to calculate with cannabis, explains Ware, because its metabolites are absorbed into body fat and then can be rereleased into the blood. But THC from smoked marijuana “rapidly dissipates,” while the effects of eating it can last 6 to 10 hours.
The key difference is in how it’s metabolized, says Ware. “Anything going through the stomach goes through the liver first,” before it gets into the blood, he explains, a process called first-pass metabolism. Cannabis absorbed through the lungs goes straight to the brain, but is only metabolized by the liver on the second pass. When the liver metabolizes delta-9 THC to 11-hydroxy THC, users feel the “combined effect of the two.”
“I don’t know if I’d say it’s more intoxicating,” Ware adds. “It’s just different.”
Why people might perceive the effects of eating cannabis to be more intense also has to do with set and setting. As the effects of smoked marijuana can be felt within seconds, even mildly experienced users can generally pick the point where they’ve had enough, a process called “titration.” In contrast, those who eat cannabis have to estimate what the dose they’ve taken will do, and the length of the high means that any unpleasant effects take longer to go away.
“People using marijuana medicinally for long-lasting chronic pain often prefer oral ingestion because it lasts longer and they don’t have to consume as often,” says Amanda Reiman, policy manager of the California Drug Policy Alliance. “However, marijuana ingested orally is more difficult to properly titrate dosage due to the increased time of effect onset.”
Predicting the effects of an oral dose is “ultimately a crapshoot,” says Armentano. Herbal drugs are difficult to standardize; this was a main reason why the medical use of cannabis, common in the late 19th century, had largely faded by the time the U.S. government enacted prohibitive laws in 1937. In addition, cannabinoids have a “unique pharmacology,” and individuals can have widely varying responses to the same dose.
Despite those variations, Ware says, once the patients he’s observed have found a dose that works, they tend to stay with that dose—but they must go through the same trial-and-error process when they get a different batch of cannabis. He recommends “the lowest dose possible to achieve therapeutic effects.”
There have been some efforts at standardization. In 2013, Colorado government officials and cannabis-industry representatives agreed to set 10 milligrams of total cannabinoids—THC, CBD, and others—as a standard dosage unit for edibles, with a maximum of 100 mg per item, says Christie Lunsford, who spent six years manufacturing and marketing edibles such as truffles and cannabis-infused soda.
The 10-mg level is a reasonable dose for occasional users, she explains, and the levels are laboratory-tested both after the plants are harvested and after the products are manufactured. However, she adds, it is impossible to measure herbal medicines with the same precision as pharmaceuticals, and having to wait for lab results means baked goods can lose freshness. In addition, all edibles sold for the state’s recreational market have to be in child-resistant packaging, and the law will be extended to medical edibles next year.
Still, if people are having unpleasant experiences eating marijuana, Armentano urges the public to keep things in perspective. Unlike those who consume too much alcohol or prescription opioids, “they’re not going to suffer any long-term potential harm, toxicity, or lethal overdose,” he says. “It’s not as if people are going to the hospital and having their stomachs pumped.”
But to avoid jeopardizing pot’s newly legal status in Colorado and Washington, he advocates that users, growers, and retailers be responsible about what they’re doing. Users need to be aware of what they’re consuming and the effects of mixing it with alcohol, he says, and those in the industry “can’t assume that people are familiar with how to use it and the effects of different doses and methods.”