A provider in the hospital calls asking for an oral tablet/capsule of dextromothorphan, preferably in a sustained release dosage form. You offer what's on your formulary (guaifenesin DM tabs) but the provider insists on this for a chronic pain patient on long-term sustained-release morphine (profile shows MS Contin 60mg BID for several months)

Why does the provider want the dextromothorphan? What antitussive effect would this added to the morphine?

It's probably NOT for cough in this case. There's some information that dextromothorphan may potentiate the analgesic effects of morphine in chronic pain, dental pain, and post-op pain patients. Some of the reports are unpublished and didn't include a placebo arm.

The pharmacology and pathophysiology are interesting. N-methyl-D-aspartate (NMDA) receptors have been shown to have some pain-potentiating effects esp in chronic pain. The neuropharmacology is probably too involved for me and this Pearl. Suffice it to say that stimulation of this NMDA receptor may 1) cause spinal neurons carrying pain impulses to stimulated with less peripheral input, 2) require less neurotransmitter to transmit pain, 3) require more antinociceptive input to stop it. Blocking the NMDA receptors, esp in chronic pain patients, may be an important future therapy. Other agents (gabapentin, alpha agonists, NSAIDs, antidepressants, antiarythmics) and pathways are being investigated in the treatment of chronic pain.

For a better and more complete explanation of this  pathways, I'd refer you to the following article:

- Brookhoof D. Chronic pain: 1. a new disease? Hospital Practice. 2000 15 Jul. 45-59.