You take it every day. It's a neural reuptake inhibitor, so it needs to build up in your system.
For me, it's not a fast fix like a stimulant would be. It just takes the edge off of my symptoms, make them easier to manage, and makes it easier to do stuff. In fact, the biggest thing I've noticed is the effect it has on executive dysfunction.
Atomoxetine (straterra) is a slow burn. It takes at least a few weeks to start noticing any effects. I haven't noticed any side effects, other than some trouble sleeping the first week when I first started taking it; but that was because my insurance wouldn't approve me for a lower dosage to have a more graceful "ramp up" period.