A federal judge has certified a class action antitrust suit that accuses GlaxoSmithKline of using monopolistic tactics to boost its profits from Wellbutrin, a popular antidepressant, by delaying a generic version of the drug from coming to market.

The suit was brought by direct purchasers who claim that GSK concocted a plan to keep Wellbutrin prices high by making fraudulent assertions to the U.S. Patent and Trademark Office and by engaging in “sham” patent litigation against generic drug manufacturers.