Where did you read this from? I think "Scientists say it may be five or ten years BEFORE it is possible to test this medicine on human patients" is more grammatically correct. In that case, I'm also confused as to why "since" was used instead of "before" :P lol

"Since" and "before" are two different things. Since: This word is used after an event. Since you called, I can't stop thinking about you. Before: Until an event happens. I eat an apple before I go to bed.

Since: after an event Before: until an event