Nearly a dozen Wesleyan University students in Middletown, Connecticut, were hospitalized Sunday after overdosing on a drug commonly known as Molly, or MDMA, on Saturday night, according to letters from administrators.

Two students remain in critical condition at Hartford Hospital on Monday evening and two others were reported to be in serious condition earlier in the day. The rest have been treated and released.

Police said 11 students were hospitalized, but Wesleyan University President Michael S. Roth wrote in a letter to the campus community Monday that 10 students and two visitors received medical treatment Sunday due to "complications arising from the use of a version of the drug Molly, a refined and more powerful form of Ecstasy (MDMA)."

Roth implored students to "stay away from illegal substances" that can prove dangerous.

"One mistake can change your life forever," Roth wrote. "If you have friends who are thinking about trying these kinds of drugs, remind them of the dangers. If you are aware of people distributing these substances, please let someone know before more people are hurt. ... Take a stand to protect your fellow students."

Calls for medical help came from the Butterfield and Foss Hill dorms, as well as 200 High Street at 7:30 a.m., 8:21 a.m., 12:26 p.m., 1:21 p.m. and 1:30 p.m. Sunday, according to Middletown Fire Battalion Chief David Anderson said. It's unclear where the students hospitalized took the synthetic drugs.

A Wesleyan sophomore listed in critical condition was one of the first taken to Middlesex Hospital early Sunday morning due to an "apparent overdose," Dean Michael Whaley, vice president of student affairs, wrote in a letter to the school community on Sunday. The names of the students hospitalized have not been released and it's unclear whether any charges will be filed.

Middletown police believe the students ingested a bad batch of Molly and are investigating to find out where it came from.

"Our first and foremost goal is to obtain information on the batch of Molly that was distributed to the students on the campus last night," Middletown Police Chief William McKenna said in a statement. "This information is critical to ensuring the recovery of those students affected."

McKenna asked students to check on their friends and to contact the public safety director at 860-685-3333 with any information.

Resident advisers, counselors and psychological service providers from the university, reachable by phone on ext. 2910, are available to students who need support, Roth said.

"And please keep those still hospitalized in your hearts and minds. Please join me in supporting their recovery with your prayers, thoughts, and friendship," Roth said. "Take care of yourselves. And let’s take care of each other."

University officials said parents of all affected students have been notified.

"Finally, I ask that you keep these students in your thoughts and share my hope that they will fully recover," Whaley wrote to the students.

MDMA stands for methylenedioxy-methamphetamine, which the National Institute on Drug Abuse says is "known as ecstasy or, more recently, as Molly," describing it as "a synthetic psychoactive drug that has similarities to both the stimulant amphetamine and the hallucinogen mescaline."

Molly has gained popularity in the last decade and has become an increasingly common concern for concert promoters, campus police and local officials.

"I think Molly is becoming a really big thing nationally and everyone is thinking, 'Oh it’s safe,' and everyone is doing it, and there could be other stuff in there," said Wesleyan senior Zaida Garcia.

A third day of Electric Zoo festival in New York City was canceled in 2013 after two young people died and four were hospitalized because of Molly overdose. Their deaths came after a string of similar overdoses that year at dance concerts in Boston, Seattle, Miami and Washington, D.C.