Mass shooters usually kill themselves negating the need for a trial. Secondarily, they plead guilty and raise no defenses.
But the point is, some of them WERE taken alive.
James Holmes (mass murderer) - Wikipedia
James Holmes never raised this defense, even though he did put his mental state on trial, he never claimed the drugs made him do it.
Jared Lee Loughner - Wikipedia
Loughner was not only not taking drugs, they had to force feed him drugs to get him to a level were he was competent to stand trial. When he was, he realized how nuts he was and took a guilty plea.
Dylann Roof - Wikipedia
Dylann Roof faced the death penalty on both the state and federal level,and never once tried to blame medications he was taking for his racist rampage.
Not one mass shooter who has been taken into custody has asserted that they were going to be perfectly fine, until those mean old drugs made them do it.
It would seem to be a pretty good defense if it has any scientific validity