The US’s Food and Drug Administration said yesterday that it has opened a safety review on reported side effects of Botulinum Toxin for the treatment of a number of non-cosmetic conditions.

Under review are Allergan brands Botox, Botox Cosmetic, and Myobloc, by another manufacturer, privately held Solstice Neurosciences.

All these products are injections of botulism toxins that block nerve transmissions and paralyse muscle. Such treatments can be used in small amounts to relax facial muscles to smooth wrinkles or calm severe spasms, such as those in patients with cerebral palsy.

The FDA said it was re-view-ing deaths and severe side-effects in children, in-cluding respiratory problems requiring ventilators and feeding tubes. The most common side-effects were in children with cerebral palsy.

In addition, the FDA said it had had reports of adult botulism cases, with symptoms such as difficulty holding up the head, weakness and numbness of the lower extremities. No adults had died and none required ventilators or tubes as a result.

The agency said that it was looking into whether these were reactions to “overdosing” and that no evidence linked the cases to “any defects in the products.”

Allergan said the most severe cases were often “severely compromised” children receiving high doses for spastic symptoms.

“We are continuing to work closely with the FDA to ensure they have all the necessary information to formulate their conclusion on the adverse event reports. We fully support the agency’s evaluation of the safety information they receive on our product,” Allergan said.

The company’s shares fell 6 per cent to $63.30 yesterday in New York trading.

Scrutiny over the safety of Botox has re-emerged in the past few weeks. Public Citizen, a US safety watchdog, recently asked the FDA to strengthen warnings on botulism products, citing deaths and fluid in the lungs and other side-effects.

Allergan has said Botox has a long safety record.

Allergan sees additional potential uses for Botox, including injections for severe headaches. It is a blockbuster product with at least $1.36bn in sales expected this year.