Ever wondered why most benzos dont really work insufflated (they only really work via the nasal drip that makes its way to the stomach in time) but many people swear by their sublingual effectiveness?
Well this could be the answer, if so
Benzodiazepines, including diazepam, are generally lipophilic (fat-soluble) and not highly water-soluble. Sublingual administration works well because the drug can dissolve in the lipid-rich environment under the tongue, where it is absorbed directly into the bloodstream through the mucous membranes. This bypasses the gastrointestinal system and liver metabolism, leading to quicker onset.
In contrast, the nasal passages rely on water-soluble drugs for effective absorption. Since diazepam and other benzos are poorly soluble in water, they do not dissolve well in the mucus lining the nasal cavity, leading to low absorption and ineffective delivery.
The nasal passages are lined with a mucus layer that is primarily composed of water. Since lipid-soluble compounds are poorly soluble in water, they may have difficulty dissolving in the nasal mucus. If a drug can't dissolve well in the mucus, it cannot be absorbed efficiently through the nasal epithelium into the bloodstream.
So essentially the mucus membranes, and the blood that is supplied to them, have different characteristics to one another. (Source, a "Reddit Drug Nerd")
I think this is some amazing info, if it's true, but personally I'm not yet convinced about the the s/l ROA, as I know the bioavailability for oral consumption is like 98% or something like that for etizolam etc, and that has got to be hard to beat.
I understand the appeal of the quick onset, especially if you already have a full stomach, but just trialling 1/4 of a crushed s/l clon now and several minutes in and not yet feeling it. (Edit: 25 minutes later and I'm only just starting to feel it slighty, But I had built up such a massive pool of saliva that I had to swalllow it eventually anyway

Further trials may be required.)
The jury is still out as far as I am concerned.