Ok, for starters... it's mainly MDMA (3,4 Mehtylene-Dioxyl-Methamphetamine Aka:- "Ecstacy") that is dubbed to be practically "worthless" to take, if you are currently on a consistent SSRI... I guess you *MIGHT* call it a "regimen".
Selective serotonin reuptake inhibitors (SSRIs) are medications that increase the amount of the neurochemical serotonin in the brain. (Remember that brain serotonin levels are low in depression.) As their name implies, the SSRIs work by selectively inhibiting (blocking) serotonin reuptake in the brain. Meaning this will in-turn, after a couple weeks of consitent SSRI administration, will cause an "over-flow" release of Seratonin in the body's Central Nervous System. Seratonin, as known to most, to be responsible for the pleasurable feelings experienced in life:- such as well-being, heightening of moods, reugulating motivation and a whole myriad of other things.
So, basically... easier said, "MDMA will more-than likely not release any mind-altering effects, if mixed with SSRI's."
With Methamphetamine usage however... and this comes from personal experience, SSRI's do not APPEAR to reduce, or "annihilate" any of the sought-after effects.
I think this is due to... (correct me if I heard wrong, do not take this as FACTUAL [alot can be found on erowid.org]) as Methamphetamine is used, it's chemical release once penetrated through the brain, will eventually cause a mass Dopaminergic release in neurons. Giving-off a feeling of Satisfaction and a rewarding experience in the user.
So, I think mainly that given that Methamphetamine is more-so steered towards Dopamine being released, not so much as specifically larger amounts of Seratonin, or other brain-chemicals as such. (I think that not as significant amounts of Seratonin, etc... are released, only minimal amounts).