Except prayer is a non naturalistic claim and doesn't really lend itself to science. All we can really do is investigate if a certain method performs better than (a) doing nothing and (b) the placebo effect.
If it does, even were we to not understand why, we would be obliged to call it medicine. The complexity is added to when it comes to mental issues since we struggle to really define functioning in that field. In that regard mindfulness, CBT, talking therapies can be partially compared with drug therapy but the clarity of results in the field of study is difficult.
The way to determine this is testing. On that basis if you accept the definition of functioning then it may well perform better than drug therapy but that does not mean it is a panacea, if there is any such thing. Further, given the complexity of this, there is a sense in which it may effectively be a form of drug therapy.
I rarely agree with Sriram on stuff like this, for example homeopathy is just snake oil to me or at least the memory of snake oil, but we are other than in a few specific circumstances, at the same stage in mental health as we were pre vaccination. (I will refrain from my opinion on the mental health of the anti vaxxers movement)