Strattera works great for some people but in clinical practice has not proven to be as great as the early clinical trials suggested.
Strattera has a high dropout rate due to any number of side-effects. Even Dr. Daniel Amen, in one of his most recent books (actually a kit) The Healing ADD Power Program says he does not typically recommend Strattera anymore due to side effects.
As Chinook says, it takes a while for the body to adjust to Strattera. If the irritability is mild, you may want to ride it out for a while. But if you are majorly concerned, do not hesitate to consult with your healthcare provider. And certainly, when you find the cost/benefit ratio of the drug slipping, it may be time to consider other options, of which there are quite a few, both med & non-med. The Amen kit, above, covers lots of the options for each subtype of ADD.
If you are due to refill your Rx & the med is not covered by your insurance, you may be able to save some money by getting only a partial refill, say I week or two week's worth, enough to get you by until you decide whether you want to continue with Strattera (then pick up the remainder of the refill), or to discontinue. Talk to your pharmacist.