Yes i have noticed that you can build somewhat of tolerance to both the H1 antagonism and anti-Muscarinic effects of the sedating anti-histamines like Diphenhydramine most of which have the side effect of being anti-cholinergics as well. Though Hydroxyzine is one that doesn't act as a anti-cholinergic but it is very sedating due to it's very strong H1 antagonism. However developing a tolerance to these meds won't help the shitty effects of higher doses at all. You will still get the restlessness, cardiotoxity and increased risk of seizures.
Diphenhydramine is as a deleriant in high doses just like Dimenhydrinate as well as some of the more lethal ones such as Belladonna, Datura and Brugmansia. The latter 3 all contain the alkaloids Atropine which has many medical uses including treating asthma attacks and Scopolamine which is used to treat nausea, vomiting, stomach cramps as a antispasmodic and motion sickness which you can get a Scop patch for called Transderm-V. These drugs are very useful in recommended doses for their medical uses but are very dangerous in high doses.
You don't "trip" on it atleast not in the way most psychedelic users use that word. Unlike LSD, shrooms, Mescaline, etc you can't tell what's real and what's not. Unlike the nice pseudohallucinations you get from Tryptamine and Phenethylamine psychedelics you don't see any colors, any pretty color patterns or experience synesthesia. Instead everything looks quite normal and alot of the time you forget you are even on any drug. You find yourself talking to people that arent there and suddenly disappear, you may think a plant is a person and start talking to it, you may see bugs (spiders seem to be a rather common hallucination on deleriants) and rats and other unpleasant things and you will feel rather dysphoric. Also these drugs cause a really nasty body loads, high blood pressure that can sometimes result in a stroke, have cardiotoxic effects that can cause heart attacks and can cause seizures. So that alone never mind the fact that it's one of the most unpleasant feelings ever is enough reason to stay away.
I was dumb enough to "trip" on Dimenhydrinate twice (i'm a sucker for punishment) when i was maybe 13. This was before we had Erowid or even really the net to look up the side effects of drugs as quickly as we can today. I had no idea that it was even a deleriant much less knew what the dose to achieve those effects where so i took 25 50mg Dramamine pills. To say it was a horror show was putting it mildly and i couldn't even sleep it off because i had Akathisia really bad all over my body so i kept twitching and lying down only made it worse.
Deleriants are very dangerous and unpleasant drugs and honestly i would say avoid them like the plague. If you want a OTC trip take DXM as unlike DPH it's fairly safe as long as you don't mix it with other meds and it can be quite euphoric. Just make sure the product contains only Dextromethorphan as other ingredients in those meds like anti-histamines, decongestants, Guaifenesin and Acetaminophen/Paracetamol can do everything from make you very sick to kill you. Another OTC high is Codeine but it's not available in the US.
