Human Growth Hormone is a prescription drug usually prescribed to children with growth disorders or adults with hormone deficiencies. It was featured prominently in the Season Sixteen episode, "A Scause For Applause".
In sports, it is also used as an anabolic agent to increase performance. This act, usually called "doping", is banned in most sports leagues and is generally frowned upon by the general public.
In "A Scause For Applause", Jesus Christ was proved to be using Human Growth Hormone, while performing his miracles. Those who believed in him realized that he did not suffer for their sins, but rather was "very high". This caused people to not only remove their 'What Would Jesus Do' scauses, but forced priests and bishops to remove Jesus from records.
After Stan Marsh and Jesus meet the owner of P.F. Pityef Bracelet Factory, the owner removes his establishment from South Park and leaves town. In an effort to restore people's faith in him, Jesus Christ uses HGH to go on a Hulk-like rampage against the factory.