Interesting as it is, my doc was the one that prescribed me both the phentermine and the AD's .... of course when I called him to tell him how i was feeling he immediately took me off the AD. it was my first and last try on those!
Both these have an effect on Seratonin, as do chocolate and carbs, our favorite comfort foods. It is interpreted in the 'feel-good' brain center.
That's a really delicate balance, as you've attested! I think because 5 mg of Prozac added to the phentermine is a popular combination doctors' choice, folks think all the SSRI's are fine with it.