a derivation of "pothead" -- one who fiendishly (i.e. compulsively) consumes medically-approved pharmaceuticals in a non-medicinal (i.e. recreational) manner
"He sure gobbles down that Percodin! What a medhead!"
A drug user, legal or otherwise
I take Advair, Clarinex, Serevent, and Singulair for my Asthma. You could say I'm quite the medhead.