Yes the placebo effect is observable in prayer/meditation maybe...

Yes the placebo effect is observable in prayer/meditation maybe from looking at the problem/request asked for from a different point of view in quite contemplation.

As for the spiritual side of prayers, the unseen results, to fulfil our purpose in this world to worship God? We'll see on Judgement/Recompense Day - so for those that believe praying to the One God may work, no guarantees in Islam, you do your best to be righteous.