Benzonatate
"I had flu with a bad cough. The other flu symptoms resolved after a few days but the cough did not. I was prescribed benzonatate as a cough suppressant. I took one capsule, and within about 20 minutes, my sinuses were clogged and congested. This means I can't breathe through my nose, which means I'm breathing through my mouth, which makes my mouth and throat dry, which causes me to cough, which is the thing that was supposed to go away with this medicine. It's done absolutely nothing for the cough, and just gave me a cold on top of flu."