Consumers who thought they were purchasing sleep aids, antidepressants and other drugs over the Internet instead were shipped a powerful anti-psychotic, sending some unwitting victims to the emergency room, federal health officials warned Friday.

The Food and Drug Administration said a number of consumers took the schizophrenia drug, haloperidol, after being shipped what they thought were a variety of different pills, including Ambien, a sleep aid, and the anti-anxiety medications Xanax and Ativan. Others thought they were getting the antidepressant Lexapro.

Preliminary analysis of the pills, packaged in plain plastic bags and mailed in envelopes bearing Greek postmarks, suggest they contain haloperidol. The FDA said it had reports of several consumers seeking emergency medical treatment for symptoms such as difficulty in breathing, muscle spasms and muscle stiffness after taking the pills.

The FDA used the occasion to remind consumers of the possible dangers of buying prescription drugs on the Web.

The FDA posted images of the suspect pills and their shipping packages on its Web site — http://www.fda.gov/bbs/topics/news/photos/haloperidol.html — to help consumers identify any suspect product they may have ordered.

Consumers apparently ordered the drugs through a variety of commercial Web sites. The FDA said it was investigating.