© 2016 The Cochrane Collaboration. Published by JohnWiley & Sons, Ltd. This review is published as a Cochrane Review in the Cochrane Database of Systematic Reviews, 2016, Issue 2. Art. No.: CD004158. Cochrane Reviews are regularly updated as new evidence emerges and in response to comments and criticisms, and the Cochrane Database of Systematic Reviews should be consulted for the most recent version of the Review.