This review is now out of date although it is correct as of the date of publication [Issue 4, 2008]. The original author team is unable to complete the update, hence the decision to withdraw.

We are seeking new authors to develop a new protocol which would serve to update the existing review and incorporate the latest evidence into a new Cochrane Review. However, we would suggest that the current topic is too broad and would therefore recommend reassessing the title prior to registration. Please contact PaPaS if you are interested: http://papas.cochrane.org/contact‐us.

The editorial group responsible for this previously published document have withdrawn it from publication.