In June 1999, popular radio personality Dr. Drew Pinsky used the airwaves to extol the virtues of GlaxoSmithKline PLC's antidepressant Wellbutrin, telling listeners he prescribes it and other medications to depressed patients because it "may enhance or at least not suppress sexual arousal" as much as other antidepressants do.

But one thing listeners didn't know was that, two months before the program aired, Dr. Pinsky—who gained fame as "Dr. Drew" during years co-hosting a popular radio sex-advice show "Loveline"—received...