WASHINGTON (Reuters) - A drug used to treat schizophrenia was mailed to some consumers who had ordered other medications via the Internet, and several users had to seek emergency treatment because they could not breathe, U.S. health officials said on Friday.

The U.S. Food and Drug Administration said that instead of receiving the products they ordered, some consumers were sent the prescription drug haloperidol. Several patients had to seek emergency care after taking the pills, the agency said without citing a specific number.

Sold legally as Haldol by Johnson & Johnson and in other generic versions, the anti-psychotic drug is known to cause muscle spasms, muscle stiffness, agitation and sedation.

Buying medication online has been controversial.

Sales of drugs from other countries over the Internet is illegal in the United States, and the FDA has repeatedly warned consumers not to buy them that way. At the same time, supporters say reputable pharmacies abroad can offer safe medications at a cheaper price.

In the latest incident, the agency said, packages were postmarked from Greece but it was not known where the pills were manufactured.

Consumers had been trying to order specific drugs online, including Sanofi-Aventis' Ambien, Pfizer Inc.'s Xanax, Forest Laboratories Inc.'s Lexapro and Ativan, sold by Baxter International Inc. and Biovail Corp.

The agency said those who have received medications from an Internet seller should compare them to photos of the faulty orders, which can be seen on the FDA's Web site at http://www.fda.gov/bbs/topics/news/photos/haloperidol.html.

"If the tablets received from an Internet seller resemble those in the photos and haloperidol was not specifically ordered, do not take these tablets," the agency said.