Yes, the Kaio-ken technique can be used by humans and other species.
While initially presented as a technique only Goku could master, the possibility of other species, including humans, learning and using Kaio-ken is implied by its nature and stated in supporting material. The technique itself is about manipulating one's ki in a specific way to enhance power and speed, something theoretically any being capable of ki control can learn.
Here's why a human could use Kaio-ken:
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Ki Control is Universal: The foundation of Kaio-ken is ki control. Characters across various species in the Dragon Ball universe are capable of manipulating their ki, suggesting that the basic prerequisite for learning Kaio-ken is not species-specific.
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Potential Unleashed is Available to All: The reference states that the Potential Unleashed state is available to all species. Since Kaio-ken involves enhancing one's existing abilities, it's logical that it's similarly accessible.
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Training and Mastery: The primary barrier to using Kaio-ken isn't inherent species limitations, but the intense strain it places on the body. Humans, like any other species, would need rigorous training to condition themselves to withstand the technique's effects.
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Example from the Source Material: While no human characters in Dragon Ball have canonically used Kaio-ken, this is likely due to factors like storyline focus and character development, rather than an inherent inability. The lack of examples doesn't negate the possibility.
Therefore, while the Kaio-ken technique is challenging and potentially dangerous, its accessibility isn't limited by species. A human, with proper training and ki control, could theoretically learn and use the Kaio-ken.
