Cystopurin Cranberry Sachets (6). Cystopurin is used to relieve the symptoms of cystitis.. The active substance in Cystopurin is potassium citrate. Potassium citrate helps to relieve the symptoms of cystitis by making your urine less acidic, reducing the discomfort caused by acidic urine.. Cystitis is a distressing condition caused by an infection and/or inflammation of the bladder. It is normally no more than a painful nuisance and only rarely becomes more serious.. Cystitis will occur in 4 out of 5 women at least once in their lifetime, so it is a very common complaint and the attacks usually cause no lasting damage.. The most common symptoms of cystitis are:. A sharp stinging sensation on passing water.. An urge to pass water more frequently