Here is a link on why Benadryl works.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8235102/
"Antihistamines act by competitively inhibiting peripheral and central histamine receptors. Antihistamines contribute to the treatment of factors affecting the onset of tinnitus. For example, their drying effect improves the functions of the auditory tubes, reduces the accumulation of effusions in the middle ear, and enhances ventilation in the tympanic cavities."

About the gelcap has double the effect of tablet & pill, I'm gonna need more info (generic/brand name).

It's just the walmart equate brand liquid gel caps, they're a pale yellow.

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and the tablet/pill?
I don't live in the US to be familiar with walmart products. If anything I need the name so I could see it's formulation, which has to do with the BA.
My first guess is the active ingredient used in the gelcap is lipophilic so it absorbs faster and can easily get through the blood-brain barrier to the active location. Whereas the one used in the tablet/pill is hydrophilic salt, so it takes a little longer to metabolise to the active form.

"Equate allergy relief softgels 25mg" is what I found by googling, seems to match op's description, dph hcl

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Idk about the CBD because the isn't many studies on its phamacology effects. But "buy the pure isolate and vape that" is a smart move to avoid dodgy products. Just be aware of the dosage.
I found this on mayoclinic: "Currently, the only CBD product approved by the Food and Drug Administration is a prescription oil called Epidiolex". You can look into this product to estimate a safe medicinal dosage.

Yeah I've tried CBD, it doesn't help me unfortunately. Have tried vaped and also water soluable tea which supposedly has 100% bioavailability which I drank every night for most of the year because it's just really good. It's chamomile CBD tea

Oh, if the gelcap has Diphendydramine HCl then it's not the case about the active ingredient but the delivery form.
When you take an immediate-release tablet, it usually takes 45-60 minutes to slowly disintergrate and release >75% of the active ingredient. That is "immediate" but still slow compare to softgel capsule.
Once a softgel's shell break down, probably in 10-15 minutes, it'll release all of the lipophilic content inside. The content is almost always lipophilic because water would breakdown gelatin. As I said before, lipophilic stuffs absorb better via GI tract (especially at the small colon), so not only the softgel release it content faster, the content itself would absorbs faster. This result in the faster peak of active ingredient concentration in blood. I think that's the reason why it feels better for OP to use the softgel form.

what kind of vape?

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Well, now I'm curious how taking it in the uncontained liquid form, Zzzquil, would stack up. And does the increased bioavailability in the gelcaps also contribute to faster elimination? Do you ever get woken up by your tinnitus?

I have, yes, but not often. I take diazepam sometimes or just some melatonin. Melatonin is stronger than Ambien for me, I'll eat like 1mg and completely crash. It can however cause difficulty falling asleep at first.

Zzzquil I don't know if I'd want to try, syrups like that disagree with my stomach a lot. But if there's one with just DPH maybe it wouldn't be so bad.

It's more than just it working faster though, I mean it straight up works way better compared to tablet or the powdered pill. Those weren't working, so I got the softgels that I'd been taking while on vacation which I'd run out of and the difference was huge.

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Absorption is effected by many factors, and varies person-to-person. So it is possible that gelcaps work better for you, but there probably isn't a good reason to suspect that this is generally true for other people or for other drugs.

Benadryl already has high oral bioavailablity (40-60% by reports). It's a little hard to get better than that, no matter the type of formulation.

In this case, faster is better though. Let's assume that your body needs X amount of the active ingredient in blood to induce the desired effect - which called the therapeutic concentration. While both the gelcap and tablet release the same amount of drug over time. The tablet may takes too long it wouldn't even reach your body's therapeutic concentration, since your liver enzymes would break it down fast enough to keep the drug concentration down. The gelcap, on the other hand, release the drug fast enough to overwhelm the breakdown rate of drug in your body, which raised the drug concentration above the therapeutic concentration, thus induce the desired effect - tinnitus gone.

You can look at graph 2.6 in this link to have a better visualisation. The green line would represent the plasma concentration released by the gelcap and the red line would resemble the tablet.https://www.pharmacy180.com/article/bioavailability-847/