Updated from 2:37 p.m. ET

Allergan

(AGN) - Get Report

said Friday it has received approval from the

Food and Drug Administration

for its glaucoma treatment

Lumigan

-- but

Pharmacia

(PHA)

doesn't plan on letting the rival pharmaceutical company get off that easily.

Hours after Allergan said it received the FDA approval, Pharmacia said it believes the patent rights covering its

Xalatan

will be violated by the use, manufacture and sale of Lumigan.

Lumigan is used to reduce elevated pressure in the eyes of patients with a specific form of glaucoma or ocular hypertension, who can't take certain medications to treat their ailments. The company said between 2 million and 3 million Americans have glaucoma, which can lead to blindness.

Earlier this month, Allergan

filed a patent lawsuit against Pharmacia in a federal court over Lumigan. Allergan said it filed the suit to clarify its position that Lumigan doesn't infringe on patents owned or controlled by Pharmacia.

Pharmacia, which is based in Peapack, N.J., said it has reviewed the specific allegations in the lawsuit and contended that Allergen's patents aren't valid.

"Based on our analysis of Allergan's complaint, we believe that the chemical structure of Lumigan violates at least two of our patents,'' the company said in a statement. "We will vigorously defend all of our patent rights supporting Xalatan and prostaglandin derivatives, and we believe that these patents will be supported by an intellectual property system designed to protect patent rights and discourage those who seek to profit from the illegal pirating of legitimate patents.''

Neither stock fared particularly well today as shares of Pharmacia fell $2.28, or 4.8%, to $45.18, while Allergan dropped $2.22, or 3.2%, to $68 in

New York Stock Exchange

trading.