It certainly looks like yet another of granny's remedies has some good scientific backing.

Although much of it is still theory and research is still ongoing, there are several possible ways in which cranberry juice may help in cystitis.

Acidifying or alkalinising urine

Some people believe that making the urine more acid can help cystitis, while others say that doing the opposite - making it more alkaline - is better.

There's some evidence that either approach calms the pain and discomfort of cystitis and controls levels of bacteria.

Acidifying the urine also prevents the formation of kidney and bladder stones.

One way to do this is to take ascorbic acid or vitamin C supplements. Drinking cranberry juice has been suggested as a natural way to acidify the urine, but to get the best effect you need a lot of cranberry, which means finding a juice with a high concentration of cranberries (more than 50 per cent - most commercial brands are only half this strength) or drinking large amounts of weaker juice.

Alkalinising urine seems to be more effective in reducing symptoms, especially for women who are 'culture negative' - that is, no harmful bacteria can be detected in their urine.

This can be done by drinking treatments based on potassium or sodium citrate, which you can buy at the chemists. It's not clear how alkalinising the urine helps but it can quickly get rid of symptoms often without the need for antibiotics.

Inhibiting bacteria

Cranberry juice contains chemicals which inhibit the activity of E. coli - the bacteria most often responsible for cystitis. The chemicals - condensed tannins or proanthocyanidins - stop the bacteria sticking to mucosal surfaces lining the bladder and gut.

This helps to clear bacteria from the urinary tract and recent research shows these may be the most important effect of cranberry juice.

The chemicals in cranberry juice may have this effect on other bacteria too. It's now being recommended as a way to reduce the risk of dental decay and stomach ulcers causes by Helicobacter pylori.

An important part of treating cystitis is drinking lots of fluid to flush out the urinary tract.

Keeping infection at bay

Two glasses of cranberry juice a day can help to prevent urine infections, especially in people who are more vulnerable, including those who have to use catheters.

Another bonus is that it can help reduce problems with the smell of urine that are a nuisance, especially for people with permanent catheters.

This article was last medically reviewed by Dr Trisha Macnair in December 2007