Any Idea As To Why Antihistamines Cause Drowsiness?

Sep 10, 2012
I don't really know anything about how antihistamines work, but if histamine is blocked, then serotonin must be reduced, increasing CO2, thus lowering the stress hormones, particularly adrenaline. Does that sound about right?


Aug 9, 2012
Histamine is involved in wakefulness, I suppose it has a larger effect if your metabolism is low. Antihistamines can also be anti-cholinergic (cyproheptadine is, cetirizine isn't), another stress hormone.


Dec 13, 2013
Antihistamines that are anticholinergic in particular reduce activation of the parasympathetic nervous system and increase activation of the sympathetic counterpart, this can cause pupil dilation, erectile dysfunction, changes in blood flow, and other "sympathetic" functions. Histamine itself, as Kiran said, is involved in wakefulness, and also arousal. I think it's healthiest to take both an inhibitor of the PSNS and SNS to reduce autonomic arousal overall. Although both should be minimized, an imbalance isn't good.

By the way, are you folks prescribed cyproheptadine, cetirizine, hydoxyzine, and other anti-histamine agents, or do you obtain it from another country like cynomel can be bought from Mexico without a prescription? Or is it over-the-counter in whatever country you're in?


Thread starter
Sep 10, 2012
Thanks for the info guys. I'm using Benadryl for now, but I plan on trying Cyproheptadine soon. I'm most likely going to buy it online without a prescription.


Feb 20, 2013
RP recommended additive free pure Benadryl and Cyproheptadine as anti-histamine.
He warned against using certain antihistamines, which has chlorine or fluorine attached
to carbon molecules. These causes liver problems .
Cetirizine and hydroxyzine have same problem.
source : viewtopic.php?f=68&t=1035&start=50#p13904


Nov 1, 2012
Ben said:
Benadryl acts as an SSRI, not sure if RP is aware of this or not.

Isn't that only in higher dosage?


Dec 13, 2013
This is a quote from wikipedia. The cited abstract on pubmed didn't have anything about benadryl.
In the 1960s, diphenhydramine was found to inhibit reuptake of the neurotransmitter serotonin. This discovery led to a search for viable antidepressants with similar structures and fewer side-effects, culminating in the invention of fluoxetine (Prozac), a selective serotonin reuptake inhibitor (SSRI). A similar search had previously led to the synthesis of the first SSRI, zimelidine, from brompheniramine, also an antihistamine.

I would be cautious about Benadryl because a lot of people have gotten irreversible issues like ringing in the ears from it. If someone has low serotonin but high histamine, it could be a good idea, but for an average person I wouldn't recommend it.


May 24, 2013
Los Angeles
Because you can't have your cake and eat it too.

Low serotonin/histamine euphoria or energy/alertness. You can only choose one.
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