It’s no secret that advertising works. Big Pharma wouldn’t spend over $4 billion a year on direct-to-consumer advertising if it didn’t mean massive profits.

What is more unknown is why drug ads that sow hypochondria, raise health fears and “sell” diseases are often the most common–and effective–even when the drugs themselves are of questionable safety.

The nation’s fourth most frequent drug ads in 2009 for were Cymbalta, making Eli Lilly $3.1 billion in one year, despite the antidepressant’s links to liver problems and suicide. Pfizer spent $157 million advertising Lyrica for fibromyalgia in 2009, despite the seizure pill’s links to life-threatening allergic reactions. The same year, it spent $107 million advertising the antidepressant Pristiq, even though it also had links to liver problems.