The figure is above Forest's market cap of $19.3B at the close of trade on Friday and compares with Actavis' $33.4B.
The acquisition would add to Actavis' recent purchase of Warner Chilcott and follow Forest's agreement in January to buy specialty-pharmaceutical company Aptalis Holdings.
One reason for the tie-up is to allow the companies to better negotiate with customers amid consolidation between U.S. Hospitals, insurers and doctors.
Carl Icahn could be set to benefit from the deal after he built up an 11.4% stake in Forest and successfully agitated for change in a bruising battle with the company's then management.