I use this for extended amounts of time when i'm cutting and i've noticed very few side effects. The first few times I took it I felt a little jittery, but that's because I never drink caffeine.
I would take it 20-30 minutes before breakfast in the morning. I'd go to the gym and do my cardio for 25-40 minutes early and it really boosts your metabolism all day long. I went from about 155lbs down to about 142-245lbs and have been strength training ever since. About every 1-2 months I do a few week cycle of cutting again and always use oxyelite. I'm not back up to about 150lbs and have cut my body fat down a ton, as well as being much stronger in the gym.