For those looking for pain relief, relaxation, or energy, the alkaloids, mitragynine and 7-hydroxymitragynine, are usually the most important elements of the kratom leaf. Mitragynine is present in much higher concentrations than 7-OH-mitragynine – an average of 66% of total alkaloid content versus roughly 2%, respectively. That said, the 7-hydroxy version is about 46 times stronger than the other and up to 13 times more potent than morphine by weight.
Here’s where it gets a little more complex. If you remember anything from chemistry class, you may recall that polarity influences the ability of molecules to pass through lipids. The hydroxy group present in 7-OH-mitra makes it more polar than mitragynine. High polarity means greater difficulty in crossing the blood-brain barrier.
The blood-brain barrier is a natural protection mechanism built into the capillaries inside the brain and designed to keep things like toxins and pathogens out while allowing beneficial substances in, though its accuracy in judgment is not perfect. The lower relative polarity of mitragynine makes it able to cross the blood-brain barrier at a rate around 10 times higher than that of 7-hydroxymitragynine, which is compounded by the former’s superior uptake by the brain.
A 2019 study found that mitragynine was actually more effective than the 7-OH version when consumed orally. That said, the study also found that mitragynine exhibits its analgesic actions as a result of the body converting it to 7-hydroxymitragynine. Once the substance is ingested, the body’s P450 enzymes effectively oxidize one into the other at a rate that would explain kratom’s activity at the mu-opioid receptors.
Side note – this is actually a microcosm of what happens when red kratom is made. After naturally green leaves are dried they are set out in the sun to cure for even longer, which speeds up the oxidation process. The color red is a result of oxidation and can be indicative of greater 7-OH-mitragynine content.
There are unfortunately very few studies covering kratom consumption by humans. The lone study to be found used kratom tea in relatively low dosage, so it can not be used to draw comparisons to animal studies that used isolated mitragynine. However, there has been research performed using dogs in order to observe kratom’s effects across species.
The first kratom dog study was carried out in 2020 by Dr. Christopher McCurdy and his team at the University of Florida. The goal was to observe the pharmacokinetics and safety of kratom in nine female beagles. One of the reasons for this choice was that dogs are the closest to humans when it comes to some liver enzymes that metabolize certain drugs.
The study found that mitragynine actually kicked in at around 30 inutes, which is about 20 minutes faster than had been reported in rat studies. This is interesting because 7-hydroxymitragynine reached its peak plasma concentrations at around the same amount of time after mitragynine consumption, so metabolization seems to occur very quickly.
Keep in mind there was no direct 7-hydroxymitragynine given to the dogs. All of the 7-OH that was present was there because the dogs bodies metabolized it from mitragynine. This effect was much more pronounced in dogs given mitragynine orally compared to the four dogs that were given it intraveneously due to enzymes present within the liver.
Mitragynine also showed an oral bioavailability of 69.6%, which is fairly impressive given that morphine’s and hydrocodone’s are about 24% and 25%, respectively. As far as drawing efficacy comparisons between the two alkaloids, it was determined that 7-hydroxymitragynine had a 22.5 times higher binding affinity at the mu-opioid receptors.
Worth noting is blood panel work pre- and post-dosage was not significantly different. Again, this study was done with dogs and not humans. However, dogs are now the second species (third if you count the single human trial) that have shown positive responses with no adverse reactions.