New York: Allergan Inc.’s wrinkle smoother Botox must carry the strictest warning about the risk of potentially deadly botulism, US regulators have ordered.

The new caution applies to all botulinum toxins used to relax the muscles that cause forehead lines or to treat spasms of neurological disorders, the Food and Drug Administration said today in a notice posted on its Web site. Doctors and patients should watch for side effects and shouldn’t interchange different botulinum products, the agency said.

Botox has about 83 percent of the global market for so- called neurotoxins, with sales of $1.31 billion last year. The FDA raised concerns about the drug’s safety in February 2008.

Another brand of the drug, Dysport, a competing toxin has also received FDA approval with the stricter warning.

“With the approval of another botulinum toxin, the FDA’s request for updated class safety labeling and the reinforcement of non-interchangeability is timely and responsible,” said Sef Kurstjens, Allergan’s chief medical officer, in a statement.

Almost 2.5 million Americans had Botox injections last year for cosmetic purposes, according to the American Society for Aesthetic Plastic Surgery, based in Garden Grove, California. The drug was approved in 1991 for medical uses and in 2002 as a wrinkle smoother. It became fashionable among aging celebrities as a nonsurgical way to appear younger.

Botulism poisoning can occur if the toxin spreads beyond the site of the injection, leading to muscle weakness or difficulty swallowing and breathing.

Most side effects have been reported following treatment of arm and leg spasms, a use not approved by the FDA, and a neck- twisting disorder called cervical dystonia. The agency ordered manufacturers to better explain these dangers to doctors and patients and to implement new programs to minimize risks.

Solstice Neurosciences Inc., a closely held company in Malvern, Pennsylvania, sells a botulinum toxin called Myobloc in the U.S. for cervical dystonia.