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I took DXM weekly for 2 years or so and went through serious withdrawal when I abruptly stopped. The withdrawal was very similar to bad SSRI withdrawal: couldn't sleep, I felt hot all the time (or maybe cold all the time, it's been a while), I had an impending sense of massive dread, I thought I was going insane..
I think the inability to sleep is what caused most of my problems.. After a couple of days I would close my eyes and see DXM-type visuals while awake, and heads squishing and morphing around and eating each other is scarier when you have emotions (not that I usually saw heads squishing on DXM normally.. I think it was the lack of sleep).
After about a week I went to a doctor and got some Paxil (an SSRI), which helped quite a bit. As the withdrawal lessened, the closed-eye visuals became less and less realistic in the same manner as my DXM trips (actual objects, growing vaguer and vaguer, then just blobs, then just colorful 'shell' things).
Eventually I got off Paxil too.. I was able to quit that cold-turkey without any problem (some very minor withdrawal, but no problem sleeping or general craziness) - I was only on 20mg a day, though, which maybe is low..
Ultimately DXM is pointless for me, at high doses I can't remember anything and at low doses I just float through featureless corridors or endlessly reach closed doors. It was fun while it lasted, though, and the few out-of-body experiences I had were neat.
I had the same visuals you mentioned, and I was also experimenting with SSRI's at the time. For me, they did not help the situation whatsoever, and often made it worse. At one point, I used 60mg (triple dose) of Citalopram to lessen my DXM withdrawals, I ended up with seratonin syndrome for that night, but the day after, I felt like I was on E. odd.