Imagine that you went to see your family physician with an ailment, and she/he prescribed some medication. After taking it, you developed a nasty reaction, were hospitalized and learned only there and then (if ever) that you should not have been prescribed this particular medication given your other conditions, age or sex. How could that happen? Well, chances are your doctor did not have access to all the information about the medicines prescribed. Oops! Or imagine that you are a researcher who spent a few years preparing a study to test a promising new medicine. You then started the clinical trial, and your study participants experienced no effects or serious side-effects. Only then did you learn (if ever) that this particular intervention had proven to be unsuccessful or even harmful in several unreported or currently ongoing studies that you were unaware of. Thus, you wasted several years on a useless or potentially harmful study.
Link to access full article: https://www.researchgate.net/publication/275966659_Sharing_of_Clinical_Trial_Data_and_Research_Integrity