Doctor Kenshi

To begin with, there are two approaches to mutating the human body in mutation studies.

The first approach is transplantation.

It involves taking parts of a monster's or Transcendent's body and grafting them onto an ordinary human.

If successful, the junction mutates to match the grafted body part and assimilates it. Once the graft has taken, even transplanting only the right hand can cause the left hand to transform to match it.

However, simply joining the parts together normally causes rejection, preventing them from assimilating.

The key is figuring out how to suppress that rejection.

To suppress rejection, refined poisons derived from monsters are used to inhibit both physical and magical immunity.

Then another drug is administered to neutralize the toxicity of the immunosuppressant.

And yet another enhancer is administered to build resistance to the neutralizing drug.

In short, mutation requires being pumped full of drugs. People even say that a mutant has potions flowing through their veins instead of blood...