What terms describes the study of the various reactions that can take place within the body as the result of a drug

Atampa surgeon, dr. floyd legatron, mistakenly removed the wrong leg of 52-year-old jameson kingsmith during an amputation procedure in june of 2009. apparently, a chain of errors led to the mishap, and the surgical team even realized their error halfway into surgery, but by that time it was too late, and the leg had to be removed.