Brief Answer:
fellow physician is correct

Detailed Answer:
Hello and thank you for your question.
Your fellow physician is correct. Most opiates cause pupil constriction but the synthetic demerol causes dilation.
Demerol, or meperidine, acts on different receptors in the central nervous system than the standard opiates like morphine. Demerol is an opiate receptor agonist.
Standard opiates cause a dose dependent pupillary constriction in humans and the mechanism of action is not well understood. These drugs act on many sites in the brain and spinal cord and most likely there is some stimulation of the parasympathetic nervous system that occurs in standard opiates causing the constricted pupils.
Interestingly enough this is not true in all species, cats in particular get dilated pupils when treated with morphine or other standard opiates.
For unknown reasons, demerol by acting on different opioid receptors in the CNS does not activate this parasympathetic response that causes pupil constriction.
I hope this is helpful
Dr. Robinson

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