"There are now three products approved in the U.S., each with different units, and the units cannot be interchanged," Unger said. "Switching patients from one to another runs the risk of underdosing or, more important, overdosing."
The three approved products are Botox, Myobloc and Dysport, which was approved this week. Officials said there have been no post-marketing reports of similar problems with the newest drug.
The injections, which temporarily "paralyze" muscles, are intended to have a localized effect but can pose problems if the compound spreads to other parts of the body.
"When given in a particular place, they spread locally, meaning they move into adjacent structures," Unger explained. "We have known that for years, and it can be annoying and somewhat of a significant problem for given patients but does not result in disability or harm."
"The real concern is when there is distant spread," he said. "It is injected at one point and spreads to areas not adjacent."
The FDA issued an "early communication" in February 2008, warning of these adverse reactions. The month before, the watchdog group Public Citizen petitioned the agency to add a black-box warning to the two drugs then on the market in the United States, describing 180 "adverse event cases" related to the drugs, including 16 deaths, four in people younger than 18.
Between early 2008 and now, the FDA became "more certain about the cases," Unger said. "We felt we really needed to nail down the scope of the problem before we placed a boxed warning, which is something we take very seriously."
The FDA will also require manufacturers to put in place what it calls a risk evaluation and mitigation strategy -- in essence, a comprehensive patient safety guide.
"Updating labeling will help patients and health-care professionals better understand the risks and benefits," Unger said. "These drugs have benefits, but they als
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