Good Question. There is conflicting evidence about cranberry products preventing cystitis. A review was published in 2008 of ten research trials that had studied the effects of taking various cranberry products each day (juice, tablets, etc). This review concluded that women who took cranberry products had, on average, fewer urine infections than women who did not take cranberry products.
D-Mannose better. The component in cranberry juice that prevents 90% of uti's is d-mannose. Taking d-mannose is a more efficient way to prevent uti's. 1/2 tsp 1-2x/day is usually effective. See http://bit.Ly/1hzudgk for details. Research does show cranberry juice can help prevent recurrent uti's but you would have to drink large amounts to get adequate amounts of d-mannose, in fact we don't know exactly how much.