Another nifty property of it is that it reduces tolerance to other substances, like caffeine. Something about calcium ions that are usually released in response to neurotransmitters, as a mechanism to cause downregulation for homeostasis -- and DXM + other NDMA antagonists blocking them.
IANAD though.
Would this help at all with withdrawal symptoms as well? I don't know anything about biochemistry so that may be a very stupid question.
If you're withdrawing from opiates for example, your best (legal) bet is to buy some Kratom online, or pick up Immodium from your local drugstore. Immodium is a weak opiate that doesn't cross the blood-brain barrier -- so it'll alleviate withdrawal symptoms, but doesn't have psychoactive effects.
If you're withdrawing from stimulants, the best thing you could get is probably Clonidine or Gabapentin, but neither of those are OTC unfortunately.
It certainly won't hurt to take DXM if you're having WD/s, regardless of the substance though.