Montelukast
"I was prescribed this for allergies, particularly for constant nasal blockage and sinus congestion.  My doctor advised that it would take 2-3 weeks to take effect. I took it for over a month and had nothing but issues. First off, it did absolutely nothing for my symptoms. In fact, it made them worse. I went from being stuffy and having sinus pressure to the point of it being very painful. I would wake up in the middle of the night so congested that my nose and sinuses hurt so badly, I had to take Advil. It made me start snoring very loudly, regardless of which position I slept in (probably due to the worsening sinus blockage). Since I stopped taking this, I no longer snore and my allergies, while not gone, are better."