For a glimpse into real-world drug use, Australian researchers went to parties where people were using a drug known as ecstasy - and discovered that users' brains were at far more risk from the drug than anyone had suspected.
The researchers also found that ecstasy pills often contain a variety of other drugs.
"What's concerning is that most studies looking at toxicity in people or animals look at a single drug," said Dr. Thomas Newton, a professor at Baylor College of Medicine, who was not involved in this study.
"We have no idea what happens when you start mixing like this."
For this study, 56 people who had taken ecstasy at least five times in the past agreed to invite the researchers to house parties where they took ecstasy once again.
The researchers collected a sample of the pills and measured users' blood levels of MDMA - the chemical that's in ecstasy - every hour for 5 hours after people took the drug. At the end of the study, each user received AUS$200 (about US$205, or 128 GBP) for participating.
In some people, the amount of MDMA reached levels that cause injury or death in primates.
The researchers found that only half of the pills consisted entirely of MDMA. The other half also contained methamphetamine or chemicals related to MDMA: MDEA or MDA.
Some pills had no MDMA at all. The ones that did had amounts that ranged widely, from as low as 25 mg to ten times that amount.
"This highlights a significant public health concern, particularly regarding the existence of pills containing more than 200 mg of MDMA," the authors write in their report of the study, which is published in the journal Addiction.
Because the research was intended to capture a realistic snapshot of ecstasy use, the number of pills people took over the course of an evening varied as well. Most users ingested more than one pill; some people took as many as five.
"Taking multiple pills is likely to lead to very high blood concentration, which may be harmful," Dr. Rod Irvine, the lead author of the study, wrote in an email to Reuters Health.
That's because concentrations of MDMA in users' blood did not stop climbing during the 5 hours of sampling.
"We were surprised that the...concentrations continued to rise throughout the study," Irvine, a professor at the University of Adelaide, said. "The higher levels are approaching those that have been shown to be damaging to brain cells in animal models."
Three users had blood concentrations greater than 700 mg/L, which was poisonous to primates in laboratory studies. Another three users had concentrations very close to that level.
"Those are big numbers," Newton said of the blood concentrations.
Irvine said that most users continued to take more ecstasy throughout the night, even though their blood concentrations from the initial pill had not peaked.
The authors speculate that users might develop a tolerance to the drug while they're using it, making them feel less intoxicated even while their blood levels of the drug are increasing.
None of the users in the study suffered any immediate health problems from taking ecstasy.
According to the U.S. National Institute on Drug Abuse, ecstasy can interfere with heart rate and temperature regulation and can cause brain damage.
Seven of every 100 twelfth-graders say they have tried ecstasy.
Irvine said that collecting data at parties is a valuable way to get a sense of what people are actually exposing themselves to.
For instance, in 14 people the amount of MDMA in the blood reached levels that had never been studied in humans in the lab.
In laboratory studies, ethical considerations prevent researchers from testing such high doses in people, so the amounts they experiment with "do not reflect the range used naturally," Irvine wrote.
Regarding the information Irvine's team collected, Newton said, "It's very unique to pull that off."
The research was funded by the National Health and Medical Research Council of Australia.