This review has been withdrawn from The Cochrane Libaray, as of issue 3, 2004. The lead author reports that the reviewers could no longer work towards updating it. Two main issues combined resulted in this decision to withdraw the review:
‐ a lack of funding to update reviews;
‐ unpublished data used in the original review were inaccessible to both reviewers, and anyone else wishing to update this review, thereby making it impossible to cite all data used in subsequent updates.

The editorial group responsible for this previously published document have withdrawn it from publication.