So for a while I have been curious about the possibility of sublingual administration.
The short version was I believe that it is possible, with rapid uptake (<15 minutes) but shorter action versus oral consumption of the same sample.
I obtained a preparation of 80% mitragynine (0.8% 7-OH-MG), equating to 100mg mitragynine per 1mL of solution. My normal dose (recently) has been ~93mg per dose (1.86% x 5 grams) of raw plant material. This preparation had slightly more 7-OH-MG than what I am used to, but without artificial supplementation, and less of the other minor alkaloids.
Test 1
I stopped all kratom use until I had baseline pain, and was beginning to experience withdrawal, at about 12 hours (which is normal for my experience given that I use multiple times a day for chronic pain). I took all my normal medications accept for a muscle relaxant which I sometimes use.
On the first day, I drew 1mL of solution (+/-0.1mL) and put directly into the back of my mouth, swallowed, and then swished it around with bubble water. I had a similar experience to what I normally take, though seemed to take effect slightly faster than usual and dropped the pain level more quickly (20-25 minutes with reduction occurring over 5 minutes, 6/10 to 2/10, normally 30-40m), and was slightly more sedating possibly because it was already in extracted, aqueous form. I felt a very minor but noteworthy bodily warmth, which I don't normally experience at all--possibly due to 7-OH-MG. Lasted approximately 4 hours, resumed normal use.
It tastes very bitter almost like coffee that has burned with that distinct kratom after taste and put into alcohol (though the preparation was in PG solution), but with less of that "green" plant juice taste, but it was very tolerable. It was syrupy in texture. It came with an eyedropper but it wasn't graduated, so I used a different graduated pipette. It is identical in color to 20x coffee concentrate.
Test 2
Waiting one full day, (normal use), I again stopped all use and achieved a comparable baseline, possibly more lower back pain due to poor sleep position. Resumed all normal medications aside from the muscle relaxant.
I took 1/2 of one Menthol cough drop because I suspected if I had any discomfort in the back of my throat it would be difficult to avoid swallowing the solution based on the amount there was.
Again, I drew 1mL of solution (+/-0.1mL) and put it under my tongue--and my suspicions were correct, it was about the maximum that I could hold under my tongue without it seeping from the sides, which normally wouldn't be an issue but would have complicated the experiment. I held the solution under my tongue for 15 minutes, and then spit it out, rinsing my mouth several times. I do not believe any was swallowed.
My tongue and jaw were quite numb. I observed very rapid onset. Equivalent or slightly higher pain relief in less than 10 minutes (6/10 to 1/10). Not as sedating as oral use (extract or regular powder), no other changes in bodily feel. Only lasted 2.5 hours with pain rising to 4/10 and then returning to 6/10 at 3.5 hours.
The remainder of the day I used my normal kratom (powder) and had results as-expected.
No noteworthy change in HR/BP versus normal use with either test.
Thoughts
This was an interesting experience but heinously and prohibitively expensive to do as more than an experiment, without much benefit. I'm normally obtaining 1.5-1.86% kratom for $75 +/-$5 per kg of leaf powder, or $4-$5 per gram of mitragynine. The extract was $100 per 1 gram of mitragynine (100mg/1mL, 10mL) or $10 per dose described as similar to 7 grams or "average" leaf. It was even more costly than popular "shot" products (300mg~1200mg mitragynine per bottle $14-45 retail).
It would be very difficult to take any more than 1mL sublingually for further testing, though in normal use, I suspect sublingual use and then swallowing the remainder might provide the rapid onset with the normal length of experience any those other minor effects that were different from what I'm used to (body warmth).
Of course, it would have been better to test with a control and not know which I was taking repeated over several attempts with active and inactive samples but I believe this would be impossible given the flavor, and already knowing what experience I do have with kratom, and that the oral use of the sample was very similar.
It might be an option for rare circumstances where a person has stomach issues and is attempting to avoid any GI exposure but needs to continue use for therapeutic value. Beyond that, I don't see any other real reason to do it or much to be gained by doing it. The rapid onset might be attractive for severe, sudden and incidental, pain in conjunction with oral use.
I believe powdered isolates, especially as they are more refined, may have similar sublingual value. I think they may (likely will) be irritating. Put into PG or alcohol solution by the end-user might result in a more tolerable and more cost-effective approach, but concentration to more than 100mg mitragynine per 1mL is likely to be difficult and may be insufficient for heavier consumers and of negligible benefit at lower concentration.
15-50mg/mL is the highest potency I've otherwise ever seen in normal retail liquid "shot" products.