Some people believe it's excess histamine causing the problem.
They've applied various anti-histamine creams to the rash for relief.
Some get the rash temporarily as toxins stored in their fat are discharged through the skin.
Doxycycline has been recommended because it has cured the rash.
I got the rash too, and it was awful. I realized that I was not having enough copper at the time, and copper is needed for histamine metabolism.
There are plenty of keto foods with copper, but I wasn't having them at that time. Foods such as, crab, lobster, chocolate, nuts, seeds, mushrooms, etc. have copper.
I still take 1mg. copper glycinate daily now because it's a better guarantee of sufficiency.