I've been taking Mucinex DM Maximum Strength once every night for about a week to stop a persistent cough (allergies) that keeps me up at night. On the plus side, it did stop my cough and helped me sleep (knocked me right out). On the minus side, it made waking up very difficult, and made me light-headed, "foggy" feeling, and made my vision blurry - all of which worsened gradually the more days I took it. 

I didn't associate the Mucinex with the symptoms I was having until I read up on it online after a fainting spell I had had over the weekend. Apparently, it can have a very negative effect...