MRSD, Delsym and some other long acting DM (aka DXM) preparations also contain polystiril, a substance that slows down the absorption of the drug. I was told by someone who is actively doing research on this class of drugs that using the long acting forms will defeat production of the "spike" of concentration of DM produced by the mini-dose that is needed.

Also, the time it is taken, just before bedtime, is important for producing a brief blocking of the opioid receptors just before the natural period of release of endorphins (2 to 4 am) the hormones that are beneficial in calming the over-active microglial cells in the PD midbrain. Too much drug, or taken at the wrong time, may cause prolongned receptor inhibition, or inhibition at a sub-optimal time for taking advantage of boosting the normal endorphin release.

This is my opinion, based on what I have learned from other scientists who have studied the process extensively in the laboratory for the last ten or twelve years.

Robert