Anecdotal: I recently started taking Milk Kefir and it has been a life changer. I never thought I had any problems till they were resolved by Kefir. Seen great improvements in energy levels and anxiety. It even fixed the minor IBS I had.
I actually make it at home as Kefir isn't sold in my country :). I couldn't yet take a call on whether its a placebo or not. My partner says its placebo effect too but I don't think so.
I bought the grains locally and use them to ferment milk for 24 hours (I live near equator so you might have to adjust for your climate). I strain the kefir - seperate out the grains - add some milk and store in the refrigerator for a week (so basically I make it once a week). I store the seperated kefir again in the refrigerator. I take one cup after lunch and one before sleeping (without any additives or even sugar).
Depends on your kefir. Kefirs are unpredictable and they CAN go bad. Just be careful with that stuff, or even let a lab analyze yours if you can. Drinking up to 50 billion organisms (in one glass) every day has the potential to shift your microbiome for the better, or the worse.