Botox injected into the bladder muscle causes the bladder to relax, increasing its storage capacity and reducing episodes of urinary incontinence, or leakage.

"Clinical studies have demonstrated Botox's ability to significantly reduce the frequency of urinary incontinence," Hylton Joffe, director of the FDA's Division of Reproductive and Urologic Products, said in a statement. "Today's approval provides an important additional treatment option for patients with overactive bladder, a condition that affects an estimated 33 million men and women in the United States."

Botox had previously been approved for other non-cosmetic uses, such as migraine headaches, severe underarm sweating and loss of bladder control due to nerve damage.

Allergan, which has yet to report full year financial results, said it expects 2012 Botox sales of $1.76 billion to $1.8 billion. Analysts have said that approval for overactive bladder could add more than $200 million a year to Botox sales.

The treatment can be repeated when the effect wears off, but with a gap of at least 12 weeks between treatments, the FDA said.

About 3.2 million Americans suffering from overactive bladder take oral medications from a class of drugs called anticholinergics, such as Pfizer Inc's Detrol. The Botox approval is for those who are not helped by, or cannot take, those drugs, the FDA said.

Overactive bladder is a condition in which the bladder squeezes too often or squeezes without warning. Symptoms include leaking urine, feeling a sudden and urgent need to urinate, and frequent urination.

Common side effects for Botox injected into the bladder reported during clinical trials included urinary tract infections, painful urination, and incomplete emptying of the bladder, or urinary retention.

People being treated for overactive bladder with Botox should not have a urinary tract infection and should take antibiotics before, during, and for a few days after Botox treatment to lower the chance of developing an infection from the procedure, the FDA said.

(Reporting by Bill Berkrot and Ransdell Pierson in New York and Pallavi Ail in Bangalore; Editing by Roshni Menon, Toni Reinhold)

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