Why does it seem like all drugs cause the same side effects?

Why does it seem like all drugs cause the same side effects?

Why does it seem like all drugs cause the same side effects?

Answers to your questions about the news.
June 17 2010 5:55 PM

Why Do So Many Drugs Make Us Dizzy and Nauseated?

Serotonin, for one thing.

"Yes, tonight, dear. I have a headache."
"Yes, tonight, dear. I have a headache."

The female-libido-boosting drug flibanserin proved marginally effective in two trials reported by the FDA on Wednesday. Still, its side effects may outweigh whatever benefits it offers: 15 percent of participants stopped taking the drug due to dizziness, nausea, anxiety, or insomnia. Why does it seem like every drug causes the same side effects?

Advertising. No one aggregates adverse event data for the 13,000 prescription drugs on the market, so it's hard to know whether dizziness and nausea really turn up more often than, say, muscle pain and anemia. However, the symptoms you're most likely to hear about are the ones associated with popular drugs that are marketed aggressively to patients. (The side effects are typically listed at the end of each commercial.) According to a 2009 Congressional Budget Office study, the pharmaceutical industry spends about one-third of its $20 billion  direct-to-consumer advertising budget on just 10 drugs. Medicines for erectile dysfunction and depression tend to get big campaigns, and they happen to cause dizziness, headaches, and nausea.

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Another factor is over-reporting of certain side effects. Patients participating in drug trials usually receive a list of possible symptoms and then check off the ones they experience at any time during the study. (The flibanserin study, like many others, lasted about six months.) The FDA requires manufacturers to list side effects that occurred significantly more often among patients taking the drug than those receiving placebo. However, for legal reasons, many companies choose to report symptoms that turn up in both the experimental and control groups. Since lots of people experience headaches, nausea, and dizziness as a matter of course, they often appear in warnings—whether or not the drug actually caused them.

This is not the case with flibanserin. Nausea and dizziness were more common among test subjects than placebo patients, for reasons that are well-understood. Flibanserin, like Prozac, Zoloft, and Effexor, interacts with your serotonin receptors. Serotonin helps regulate the stiffness of the blood vessels in the head, and anything that affects this function can produce dizziness and headaches. (Migraines are strongly influenced by serotonin.) There are also serotonin receptors in the part of the brain that controls the sensation of nausea and induces vomiting.

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Explainer thanks Karen Riley at the FDA and Bryan Roth at the University of North Carolina and the NIMH Psychoactive Drug Screening Program.

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Brian Palmer covers science and medicine for Slate.