Anecdotally, I've used it on and off for about 10 years now and personally notice a strong correlation with impulse control. I have an OCD diagnosis and the only two things that have worked to manage symptoms are consistent journaling, exercise, yoga, meditation and therapy (all together), and 5-HTP.

This comment sums up why I’m so sad to be living in what I feel will be looked back on as the dark ages of neuroscience - these very very smart people are limited to “more correlated brain activity might help with everything!”

OTOH perhaps I don’t want to live in a world with happy pills… we should all be grateful

I can't speak for any other illness mentioned, but from what I've been told as someone with ADHD a significant part of the "nature" side of it is caused by either reduced serotonin sensitivity, a faster-than-normal re-uptake of serotonin whenever it gets released (meaning it doesn't get to do its job), or some other disruption of how serotonin is supposed to function in the brain.

So when it's caused by reduced sensitivity I would expect it to potentially be a treatment for that part of the illness, yes.

I wouldn't be surprised if it eventually turns out that ADHD has multiple potential causes that all lead to the same symptomatic serotonin disruption though, and that only a limited number of people who have it get benefits from this treatment.

And of course, that's just the "nature" side, as mentioned. There is also a "nurture" side of things where not getting the right help to deal with the consequences of ADHD leads to issues that need to be solved through therapy (although I suppose effective medication does make it easier to able to endure therapy).