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- Mar 19, 2004
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I was doing some light research the other day after assisting my (paramedic) partner in giving diphenhydramine (benadryl) and hearing his warnings about the potential side effects of the drug, one of which is sedation. I understand that the drug works primairly as a H1 receptor antagonist, but also that it is quite non-selective and has some potent anticholinergic effects as well. Supposedly it is this effect that is the cause of the sedation (and many of the drug's other side effects also). Same is the case for Promethazine/Phenergan, another anticholinergic/sedative.
What I dont understand is why inhibition of the cholinergic (parasympathetic) response through reduction of acetylcholine produces sedative effects. It is my understanding that the parasympathetic response IS the relaxed, "feed and breed" sedate kind of response. It seems to me that inhibition of this system would serve to rather excite the patient, not sedate. (...Like Atropine will cause an increased heart rate, not decrease it.) Can anyone explain to me what mechanism is working here? I know there are some tox people in here, and many others much more knowledgable than myself: please help me understand!
RE:
diphenhydramine: http://en.wikipedia.org/wiki/Diphenhydramine
promethazine: http://en.wikipedia.org/wiki/Phenergan
anticholinergic: http://en.wikipedia.org/wiki/Anticholinergic
What I dont understand is why inhibition of the cholinergic (parasympathetic) response through reduction of acetylcholine produces sedative effects. It is my understanding that the parasympathetic response IS the relaxed, "feed and breed" sedate kind of response. It seems to me that inhibition of this system would serve to rather excite the patient, not sedate. (...Like Atropine will cause an increased heart rate, not decrease it.) Can anyone explain to me what mechanism is working here? I know there are some tox people in here, and many others much more knowledgable than myself: please help me understand!
RE:
diphenhydramine: http://en.wikipedia.org/wiki/Diphenhydramine
promethazine: http://en.wikipedia.org/wiki/Phenergan
anticholinergic: http://en.wikipedia.org/wiki/Anticholinergic