Yeah kratom does indeed contain one antagonist although in such small quantities IDK if it even has an effect, I personally do believe it's the main reason my tolerance to the opioid effects seems to build slow and I can relate to the short period of withdrawal after dosing high with a high tolerance. Lol nexus should really start paying me cuz I post this link all the fucking time, it's just a great page regarding the alkaloids so I guess this is the page I'm gonna be posting to the day I die...
Corynantheidine: μ -opioid antagonist, also found in Yohimbe. < 1% of total alkaloid content found in Kratom leaf.
Source:
https://www.dmt-nexus.me/forum/default.aspx?g=posts&m=488576
When I would go on Kratom binges, I would randomly experience a sort of precipitated withdrawal shortly after dosing. This would happen maybe 10% of the time I dosed and lasted maybe an hour or two before gradually declining.
Yeah I guess I can co-sign with this, I've always thought the antagonist played a role in kratoms ceiling effect and at some point tolerance rises to far and the antagonist takes over but I know very little about the actual pharmacology of this mysterious plant. I also think another explanation would be that eventually the amount of stimulant and other alkaloids gets to a point where it just makes you anxious and trembly, kinda like too much coffee but since your tolerant to the opioid effects the whole experience feels rather rough and unpleasant so I can agree with flaga's theory, It's probably the most valid one being that the antagonist makes up less than 1% of the alkaloids but then again if coryantheidine is very potent by weight that less than 1% could be plenty enough.
Another thing I noticed was that Kratom made my Norco completely worthless (would take several days of no Kratom for me to feel the effects again). I had originally thought the Kratom made my opiate tolerance skyrocket until I dosed the Norco after abstaining from Kratom for 3-4 days and it hit me like a truck
Couldn't have been more off in my case, I've always wondered why kratom never rose my opioid tolerance. I give myself one off day a week and 30-40mgs of hydro or oxy or half a point of tar still gets me high, these are doses I was using with no tolerance before I got on kratom.
I really think Kratom has very strong, long acting antagonists but in very small concentrations. When you go on a Kratom binge, the antagonist builds up to a point of saturation (you experience the precipitated withdrawal for a short time).
Again my experience has been the opposite, I should mention that I'm in no way discounting your experience, just sharing mine... I feel obligated to say that now as some people think I'm just shitting all over their ideas and get really defensive

. If this antagonist is active I believe it's very short acting as I have taken an opioid right after my kratom dose and have never entered any half way withdrawal state like I do from kratom. This makes me believe that the agonist and antagonist are both short acting and only last for the duration of the kratom high but this is mere theory with no proof. It may just be those extra alkaloids that are causing these spikes in anxiety and tremors in high doses, kratom is related to yohimbe IIRC which has unpleasant effects in high doses.
I also think your tolerance to opiates is much lower than you realize while using Kratom. The long half-life of the antagonists creates the illusion that tolerance has sky-rocketed while in reality, this is what's keeping your tolerance down (think about how quickly the effects of Kratom return when you abstain for a few days or in my case, getting a full blown opiate effect from 50mg of hydro within a few days of feeling absolutely nothing from the same dose).
The bolded part is easily explainable by it's super short half-life, I believe the HL is like ~3 hours only and this is short enough for me to not cause any suspicion for these short lasting withdrawals and tolerance effects. The first part is a very interesting theory though I just can't subscribe to the fact that the antagonist, if even active at the doses that are in kratom, is indeed long acting for the reason I mentioned above but I could be wrong.
Additional evidence is the Kratom withdrawal. How many times have you heard the Kratom withdrawal described as "very forgiving physically when compared to traditional opiates but with a difficult mental component"? I'm thinking the mental aspect of Kratom withdrawal is caused by one of the other alkaloids that perhaps has SSRI-like action. The physical aspects of Kratom withdrawal are then nothing more than mild opiate withdrawal that results from cessation of use with a relatively low tolerance.
Again your kinda losing me, yeah many report a difficult mental withdrawal (depression, anxiety, extreme cravings) but this is a characteristic seen with all opioids and if there were any significant SSRI activity we would be seeing symptoms like brain zaps as well.
Maybe kratom has some sort of SSRI action but I haven't seen enough evidence to sway me. Let's take a look at tramadol, the withdrawals from tramadol, which has confirmed SSRI/SNRI activity, are devastating for most and have been proclaimed to be far worse than even your standard opioid like hydro or oxy, something I just can't subscribe to.
If kratom does in fact have any sort of SSRI or SNRI activity (which it may very well) it's rather mild at best. I should also mention that I don't get many mental effects other than anxiety but I'm diagnosed with GAD so it's no surprise I'm thrown back into my old mindstate x2 for the duration of withdrawal but no significant depression though your right, this is a side effect seen by many kratom users but also many opiate/opioid users as well. I personally find kratoms withdrawals very mental but in another way, I find them mental as in if I keep myself buisy and my mind occupied the withdrawals are laughable but if I'm chilling at home by myself or trying to get sleep the withdrawals kick in full force, the only other poster I've seen bring this up is Seatle Stranger though so I may be in the lucky minority here so YMMV.
++++++++++++++
Mitragynine: Indole alkaloid. Analgesic, antitussive, antidiarrheal, adrenergic, antimalarial, possible psychedelic (5-HT2A) antagonist. Roughly 66% of total alkaloid content found in Kratom leaf.
This is off-topic but I've always found this very fascinating, has anyone else ever got really mild psychedelic effects after smoking cannabis on kratom? I've noticed this very mild but colorfull honeycomb pattern that overlays parts of my TV screen when really high on both. Also, I'm a regular psychedelic user and it's by far the class of drugs I hold the most repect for and favor and I noticed that opiates/opioids kill or dull my trips but kratom seems to synergize and only dull some trips very slightly, maybe mitragynine is the reason for this, it is in fact an indole like some psychedelics.