After an apparent overdose of the popular synthetic party drug “Molly,” at least 11 students from Wesleyan University were hospitalized, with two critically ill, police said on Sunday.

Seven of the students were taken to Middlesex Hospital in Middletown, Conn., on Sunday, while another four were able to get themselves there on their own. Soon after, the two in critical condition were sent by helicopter to Hartford Hospital, the NY Daily News reported. Two others are in serious condition.

Late Sunday, Middletown Police are investigating the source of the Molly, which is the supposed “pure crystalline powder form of MDMA, also known as ecstasy, according to the National Institute on Drug Abuse (NIDA). “Our first and foremost goal is to obtain information on the batch of Molly that was distributed to the students on campus,” Police Chief William McKenna told The Associated Press. “This information is critical to ensuring the recovery of those students affected.”

The symptoms the students exhibited were consistent with a Molly overdose, school officials said. However, McKenna said it could have been a “bad batch” of the drug. Not only can the man-made drug vary in potency, but dealers will sometimes cut the drug with somewhat cheaper substances to turn a higher profit, too. According to NIDA, these substances can include methamphetamine, synthetic cathinones (bath salts), caffeine, cocaine, and ketamine. Overdose symptoms include sharp increases in body temperature, which can cause liver, kidney, and cardiovascular system failure — death is a worst case scenario.

Reports of the overdoses first emerged early Sunday morning in an email from the Vice President of Student Affairs Michael Whaley; he reported three cases. Later, in the afternoon, another email brought that number up to six, and by about 6 p.m., the total was raised to 11. Ambulances were on campus throughout the day to ensure any students needing medical attention got it promptly. However, by 5 p.m., reports to the fire department had slowed down, the university’s newspaper, The Argus, reported.

“I think that’s why it’s so shocking; because it feels like that could never happen to anyone that you know,” Emma Soloman, a freshman at the university, told NBC Connecticut. “It’s like no one is going to overdose, you know? Because it’s so common, but then when it’s in that grand of a scale, it’s scarier.”

As a stimulant and hallucinogenic, MDMA induces feelings of energy, euphoria, and emotional warmth. These effects are a result of a speedier heart rate and higher blood pressure, as well as increased activity among the neurotransmitters serotonin, dopamine, and norepinephrine, which then trigger the release of oxytocin and vasopressin — both play roles in love, trust, and sexual arousal. Regardless of these feel-good effects, it’s practically impossible to determine the purity of the drug without an at-home drug test kit. Of course, your best bet is to just avoid it entirely.