Story highlights Cough syrups perform no better than a placebo in many studies

They do have one known benefit: They can make you drowsy and help you sleep

No medicine cabinet is fully stocked without some good old cherry-flavored cough syrup -- or at least that's how it feels during cold and flu season.

But does that stuff even work?

In theory, antitussive drugs (like Dextromethorphan, often referred to as DM or DXM) block the body's cough reflex. Other common cough syrup ingredients include expectorants that thin mucus and make it easier to cough; decongestants that open airways by narrowing blood vessels; and antihistamines that reduce swelling in the nose and throat.

But despite all that, most studies on the topic have found no evidence that over-the-counter cold medicines actually suppress or stop coughing. In many cases, they've performed no better than a placebo.

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