Allergan Inc's wrinkle-filler Botox and a similar product must carry a strong warning about the risks of severe complications if the injected botulinum toxin spreads in the body, U.S. regulators said on Thursday.

The Food and Drug Administration said the move followed the investigation of deaths and hospitalizations of children, most of whom were given one of the products to treat cerebral palsy. Most harm in adults occurred after treatment of spasticity or cervical dystonia, the FDA said.

The agency said "no definitive serious adverse events" related to toxin spread had been identified with dermatologic use of Botox at specific approved doses.

Deaths were reported in adults, but "it is not possible to attribute them to the botulinum toxin because the patients also suffered from complications of their pre-existing conditions," the FDA said.

Allergan shares were down 3.5 percent to $45.65 in afternoon trading on the New York Stock Exchange.

The FDA said the information must be highlighted in a boxed warning, the strongest type for prescription drugs. The action applies to Botox and Myobloc, made by Solstice Neurosciences Inc.

Allergan said in a statement the company would work with the FDA to update the label for Botox. An official at Solstice Neurosciences could not immediately be reached.

Earlier on Thursday, Medicis Pharmaceutical Corp and Ipsen announced they had received FDA approval for a rival product called Dysport. The companies said they also were asked to include a boxed warning about the risks if the toxin spreads.