May I ask for what condition you got the memantine scripted? Did you specifically ask for it?
I discovered it by coincidence, cause low-dosed dissociatives have an unique antidepressant, motivating and mood-stabilizing effect on me and I wanted to know how it worked… then I was amazed to see that a "dissociative" is actually available by regular prescription, even though it was very expensive back then and I had to acquire it from India. So I found out that memantine isn't really dissociative, one may feel distant and numb when dosing too high but it's really a medicine and not a recreational drug. A few have tried it (see the quote in this threads first post) and it's not fun but a way to have a very valuable substance scheduled.
For me its a good 'neutral' (vs. 'depressing' ones like the anti-manics) mood stabilizer, anxiolytic and synergetic with methylphenidate (much less rebound, tolerance, side effects in general).
I'm now the third year on it and notice no real side effects, recent bloodwork showed nothing unusual too. But I feel that taking memantine alone is a waste, and it made me probably a bit lethargic even when I did not realize it. Where it really shines is combined with methylphenidate.
And opioids. While I somewhat decided that opioids don't work as expected for me (continuous use makes me anxious and depressed, which led to combined stim use until I realized that with only the stim and memantine I feel better…), it still amazes me how on memantine there is few tolerance at all, and withdrawal was repeatedly completely painless even with very few tapering, due to the lack of tolerance my doses weren't escalated but still- from an experience with naloxone I know what people usually go through.. After the opioids had been flushed out, I realized to actually enjoy to be sober and feel the dopamine again. No cravings at all. I can and do still use low-potency opioids from time to time to relax, as they don't shut me down like benzodiazepines would. But when available, I prefer ketamine to any opioid… (btw, I have combined memantine with most of the dissociatives and quite a few other drugs, it appears to have no bad interactions, but take this with a grain of salt.)
In NSPD some warn about possible long-term consequences, afaik it's still debated if and how the NMDA receptors react to continuous blockade. But from all what I've read yet, memantine appears to be quite safe as it has a low affinity and does not block the NMDA pathway completely in therapeutic dosages.