There are so many different acai berry products available, although not all will do what you want. You have to be very careful when ordering an acai berry pill as some are produced differently to cut costs, this does however result in them being less effective.
First you should always stay away from FREE trials, there is no such thing as FREE! They will ask you to pay the cost if shipping, although every month you will be rebilled and this is VERY hard to cancel.
You also need to stay away from "cheap" pills, these will not contatin pure acai berry and therefore they will not be as effective. The actual Acai Berries are only grown in one area, therefore they are very hard to harvest and are not cheap to produce. If the pills are being sold for a couple of dollars a bottle then they really will NOT be effective.
