More than 500,000 emergency injection kits for treating severe allergic reactions and asthma attacks are being recalled in the United States and Canada because the drug may not work.

The kits are routinely carried by people who are prone to acute asthma attacks or have deadly allergies to certain foods or bee stings. Users inject themselves with epinephrine, also known as adrenaline, to counter the reaction.

The maker of the epinephrine, American Home Products' Wyeth-Ayerst Laboratories, said routine quality-control tests found that the drug was not as potent as it was supposed to be.

The recall is just the latest setback for American Home, which faces legal problems over its diet drugs and its Norplant contraceptive device. Shares of American Home, down more 40 percent this year, fell 56 1/4 cents today, to $41.56 1/4, on the New York Stock Exchange.

The recall involves three kits: the Insect-Sting Treatment Kit packaged and distributed by Derm/Buro Inc., and the Hollister Stier-Ana-Kit and the Ana-Guard, both packaged and distributed by Bayer Corp.

All of the kits are sold by prescription and cost around $40.

The kits should be returned to the hospital, clinic or pharmacy where they were obtained, American Home said.

Philip de Vane, North American medical director for Wyeth-Ayerst, said people should take no chances and should contact their pharmacists or health care providers to determine whether their kit is being recalled.

Consumers can call Wyeth-Ayerst at 1-800-999-9384.

About 450,000 Ana-Kits and 55,000 Ana-Guards are being recalled in the United States and Canada, said Kathleen Bauernschmidt, spokeswoman for Hollister-Stier Laboratories of Spokane, Wash. Bayer sold its allergy kit manufacturing business to Hollister-Stier in June.

Last week, American Home agreed to pay a reported $50 million-plus to settle claims from 36,000 women who complained that Norplant caused headaches, irregular menstrual bleeding, nausea and depression.

In August, a jury awarded $23 million to a Texas woman who claimed the company failed to warn her about the dangers of American Home's diet drugs Redux and Pondimin, which is part of the drug combination called fen-phen. Hundreds of lawsuits are now pending nationwide against the company over the drugs' potential to cause heart-valve problems.

The company this summer suspended shipments of a childhood diarrhea vaccine after it was linked to bowel obstruction in 32 infants. And last year, American Home recalled a painkiller Duract after it caused fatal liver problems.