Phenergan and Benadryl

[VJWDO]

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Does anybody know why Benadryl is given to patients who are demonstrating extrapyramidal effects from Phenergan. Just curious from the pharmacist stand point.

 

[kwizard]

Phenegran (promethazine) is basically one of the earliest antipsychotics developed backed when the agents were originally developed for a desire for a new class of anesthetics. Phenegran and other similar agents like compazine are used primarily as antiemetics now a days due to their affinity for blocking dopamine at chemo receptor trigger zone (at doses much lower than antipsychotic doses).

So the promethazine blocks dopamine like any other antipsychotics (in particularly in the nigrostriatal region of the brain). When you have a deficiency of dopamine in the nigrostriatal pathway (whether it to due to drug effect w/ antipsychotics or dz state like in Parkinsons) you get extrapyramidal symptoms, tardive dyskinesia, etc. You'd give benadryl (or any other anticholinergic like cogentin (benztropine)) to offset the deficit of dopamine in the nigrostriatal region by also decreasing the amount of acetycholine thus trying to bring balance back to the system thus resolving and/or minimizing EPS symptoms (more likely to minimize vs resolving). This explanation is kind of oversimplified but hopefully it answers your question. If you have access to some pharmacotherapeutics texts (Koda-Kimble or Dipiro) and/or Pharmacology Texts (Goodman & Gilman) either of these would be able to explain the mechanism in further depth.

 

[sdn1977]

Phenegran (promethazine) is basically one of the earliest antipsychotics developed backed when the agents were originally developed for a desire for a new class of anesthetics. Phenegran and other similar agents like compazine are used primarily as antiemetics now a days due to their affinity for blocking dopamine at chemo receptor trigger zone (at doses much lower than antipsychotic doses).

So the promethazine blocks dopamine like any other antipsychotics (in particularly in the nigrostriatal region of the brain). When you have a deficiency of dopamine in the nigrostriatal pathway (whether it to due to drug effect w/ antipsychotics or dz state like in Parkinsons) you get extrapyramidal symptoms, tardive dyskinesia, etc. You'd give benadryl (or any other anticholinergic like cogentin (benztropine)) to offset the deficit of dopamine in the nigrostriatal region by also decreasing the amount of acetycholine thus trying to bring balance back to the system thus resolving and/or minimizing EPS symptoms (more likely to minimize vs resolving). This explanation is kind of oversimplified but hopefully it answers your question. If you have access to some pharmacotherapeutics texts (Koda-Kimble or Dipiro) and/or Pharmacology Texts (Goodman & Gilman) either of these would be able to explain the mechanism in further depth.

Agree w/Kwizard here...promethazine now is normally used as an antiemetic, and in my experience mostly with migraine pts. Chemo & nausea associated w/ multiple pregnancies do much better on Kytril, etc... However, sometimes, you have a pt who has been given phenergan & has some EPS symptoms - diphenhydramine is really fast. Also, in some circumstances, diphenhydramine in a low dose can potentiate the sedative & antinausea effects of promethazine so can be used for sedation in younger pts (there really are better alternatives for this too!) In an ER situation, injectable diphenhydramine is fast, predictable & reliable in reversing EPS (also...nurses can find it faster in Pyxis than benztropine, if its even there, IMO....)

 

[alwaystired]

Agree w/Kwizard here...promethazine now is normally used as an antiemetic, and in my experience mostly with migraine pts. Chemo & nausea associated w/ multiple pregnancies do much better on Kytril, etc... However, sometimes, you have a pt who has been given phenergan & has some EPS symptoms - diphenhydramine is really fast. Also, in some circumstances, diphenhydramine in a low dose can potentiate the sedative & antinausea effects of promethazine so can be used for sedation in younger pts (there really are better alternatives for this too!) In an ER situation, injectable diphenhydramine is fast, predictable & reliable in reversing EPS (also...nurses can find it faster in Pyxis than benztropine, if its even there, IMO....)

are both considered antihistamines? i will dig up my notes that i should have studied this week.

 

[sdn1977]

are both considered antihistamines? i will dig up my notes that i should have studied this week.

Yep...promethazine is a phenothiazine type antihistamine and diphenhydramine is an ethanolamine type. Because of that they have differing antihistaminic, anticholinergic & antiemetic effects...thus differing uses.

 

[alwaystired]

Yep...promethazine is a phenothiazine type antihistamine and diphenhydramine is an ethanolamine type. Because of that they have differing antihistaminic, anticholinergic & antiemetic effects...thus differing uses.

thanks