Allergan has agreed to pay $25 per share in a deal to buy neurology group MAP Pharmaceuticals.
The per share cash offer represents a 60% premium over MAP's closing stock price on January 22, and amounts to around $958 million.
The definite merger agreement, expected to complete in the first or second quarter of this year, has already been unanimously approved by both companies' boards of directors.
Allergan, the Irvine, USA-based maker of Botox, is hoping that the move will strengthen its foothold in the migraine arena.
MAP's flagship orally inhaled drug Levadex is currently under review by US Food and Drug Administration as a treatment for acute migraine in adults, and if approved will complement Allergan's own migraine treatment Botox.
The firm already partnered with MAP on Levadex in $157-million co-promotion deal back April 2011, but merging with the firm will give its fuller access to the drug, and its other potential indications, such as cluster headaches.
On the other side, according to MAP's president and chief executive Timothy Nelson, his firm's acquisition by Allergan "will increase the potential for our product candidates to make a meaningful difference for patients we have worked so hard to serve".