Irvine-based drug maker Allergan Inc. said Wednesday that the Food and Drug Administration approved its flagship Botox for treating a form of overactive bladder.

The FDA said that Botox can be injected into the bladder to treat patients who lose bladder control because of nervous system damage resulting from conditions such as multiple sclerosis or spinal cord injuries. Overactive bladder causes frequent urination, the urgent need to urinate and an inability to control urination.

Allergan just got a similar approval from European regulators earlier this month.

One injection of Botox can relax the bladder and increase its storage capacity, an effect that lasts for about 10 months, Allergan said.

The drug maker will try to gain approval for a broader population of patients with overactive bladder, said Allergan Chief Executive David Pyott.

Analysts project that Botox could gain up to $500 million in annual sales if regulators approve a wider overactive bladder label, Pyott said.

Botox has annual sales of about $1.5 billion, about half of which comes from its cosmetic use for smoothing wrinkles.