Shares in Somaxon Pharmaceuticals have rallied following an announcement that the company has signed a co-promotion agreement with Procter & Gamble for its Silenor medication.

Under the terms of the agreement, Procter & Gamble will promote the insomnia treatment to primary care physicians, while Somaxon will focus on specialists.

Somaxon has also granted Procter & Gamble a right of first negotiation to develop and market the drug as an over-the-counter medication in the US.

Following the announcement, shares in Somaxon rose 32% to $4.43.