Kratom contains a large variety of different alkaloids.
Many can provide numerous positive effects, but the two most impacting of these are 7-hydroxymitragynine and mitragynine.
Both of these alkaloids, while different in some respects, are both responsible for the painkilling effects of kratom.
Mitragynine is by far the most abundant of all alkaloids found within the plant’s leaf and has a variety of different properties, which characterize the effects of kratom leaf.
The effects of mitragynine occasionally vary slightly from person to person, depending on their own sensitivity. However, overall, the effects of mitragynine are fairly stable, and have been used for:
The two most characteristic effects of mitragynine, however, are pain relief and stimulation.
While not as potent as its alkaloid cousin, 7-hydroxymitragynine, this alkaloid does have some pain relief potential.
There are some strains that are low in 7-hydroxymitragynine such as white vein varieties of kratom and Thai strains that still have decent painkilling properties thanks to high levels of mitragynine.
Interestingly, unlike other painkillers, mitragynine shows no evidence of respiratory depression, the main cause of death in opiate addicts.
One of the most characteristic effects of mitragynine is stimulation.
In strains that give of energetic effects, you will always see a moderate to high level of mitragynine.
Combined with this alkaloid’s pain relief potential, you will find the energy given from this alkaloid is typically less jittery than other “pure” stimulants, such as caffeine.
One of the most interesting things about mitragynine is that the effects tend to change depending on dosage. Typically, at lower dosages, the effects tend to be more stimulating, whereas higher dosages tend to be less stimulating and more relaxing and painkilling.
As you may have noticed, mitragynine has several different properties, unlike most other alkaloids which have only one effect. The reason why this is can be found in the pharmacology of the alkaloid.
Mitragynine effects three different receptor sites within the brain, the most prominent of these are the adrenergic receptors, responsible for the stimulating effects via release of adrenaline.And the its action at mu-opiod receptors isresponsible for its painkilling effects.
Mitragynine is a very unique alkaloid and much research is still left to be done to understand its full mechanism of action.
While there is typically only very little of this alkaloid found in plain leaf, it is by far the most potent.
Unlike mitragynine, this alkaloid has a much smaller area of action, but where it does act, it is highly effective.
This fact is one of the main reasons why so many vendors are attempting to make 7-hydroxymitragynine kratom tinctures, with more or less success.
In terms of pain relief, 7-OHM is much more powerful compound than mitragynine, but with a much shorter duration of action and half-life (2.5 ± .7 hrs) 
However, despite this, it still has powerful painkilling effects and very little of the negative effects found in traditional painkillers.
The hallmark of 7-hydroxymitragynine is its powerful painkilling effects. Unlike mitragynine, only very small amounts of this alkaloid are needed to lessen even the worst pain.
Similar to mitragynine, 7-hydroxymitragynine effects much of the same same receptor sites, but is much more potent by weight than mitragynine, and also has little to no directly stimulating effects.
In all the best kratom strains for pain, you will be sure to find high levels of this alkaloid.