Forest and its Danish partner H. Lundbeck won the case against Ivax Corp, a subsidiary of Teva, after it applied to the FDA for approval to sell a generic version of the drug in 2003.
More recently, Forest and Lundbeck have launched similar legal action against Caraco Pharmaceutical Laboratories after it applied for approval for its own generic of the product.
Lexapro is an important product for Forest with sales of over around $2 billion last year. As such, shares in the company rose over 15% in the wake of the verdict.
Although Teva has not yet commented on the ruling, it may decide to launch an appeal.