Headaches, stuffy nose. It takes a few hours to kick in and the boners kind of feel not quite natural and it sometimes simply hasn't worked to satisfaction. It's kind of odd. Sometimes it seems to last a couple of days and other times it seems to last 12 hours, tops.
I recently tried the Levitra which worked well and within 20 minutes. I have some of their Viagra, too, but haven't tried it yet. I've seen it suggested that you can acquire a tolerance to these compounds and rotation was recommended.