Mutant Growth Hormone
Mutant Growth Hormone or MGH (and less often, MGN ) is a fictional drug in the Marvel Universe.
The drug is extracted from genetic material taken from superpowered individuals, and is usually presented in pill form. Note that, despite the name, the drug is not limited to being refined from mutants. It can be refined from most superpowered individuals, if their genetic make up has been altered.

# Effects
MGH grants temporary powers to whoever takes it, described as a "temporary genetic shift". Supposedly, these powers are duplicates of those belonging to whomever the genetic material necessary to make the drug was extracted from, although this seems inconsistent. Usually, when the source is unspecified, the effect is increased strength and aggression. If someone who already has superhuman powers takes MGH, those powers are enhanced.

# History
The first appearance of MGH is in Amazing Adventures (volume 2) #11 created by Dr. Hank McCoy, also known as the X-Man Beast.  Hank McCoy discovered what would later become MGH while working for the Brand Corporation, but consumed it in an effort to protect his fledgling discovery from a rival scientist.  The (not yet addictive) drug caused Beast to grow gray fur (which later turned blue) all over his body and acquire sharp ears, elongated canine teeth, claws, enhanced senses and an accelerated healing factor.  When he could not reverse the mutation, McCoy came to accept his fate, though his actions continue to haunt him to this day.
One supposed appearance of the drug appears in Iron Man vol.3, #3334, a drug was passed around to a party crowd which gave them all very familiar powers and caused them to run amok. The drug was not named here, however it could easily have been MGH. A drug with very similar uses was being sold by the Vanisher in the 2001 annual of Uncanny X-Men not soon after.
MGH came to prominence in comics written by Brian Michael Bendis. It was central to the plots of the "Lowlife" story arc in Daredevil, in which the Owl was refining his own genetic material and building a criminal empire by dealing it (his dealers claimed it was from Spider-Man).
In the "The Underneath" arc of Alias, Mattie Franklin, the third Spider-Woman, was exploited as the source of the material.   This storyline is being followed up in The Loners miniseries, in which Mattie, Darkhawk and Ricochet target the MGH dealers and the mutant Nekra.
Patriot of the Young Avengers was revealed as a user of MGH. Having no superpowers of his own, he misled his teammates into believing he had inherited his grandfather Isaiah Bradley's super powers, when in fact Patriot was regularly taking MGH in order to be as effective as his teammates in battle.  (He took the drug from street distributors and manufacturers whom he apprehended.)  When this secret was revealed, he quit the drug and the team, though he later rejoined, obtaining powers similar to the ones he claimed from a blood transfusion by his grandfather.
The Runaways have also encountered the drug on their jaunt to New York City. Cloak, of Cloak and Dagger, was pursued by the New Avengers after being caught on tape savagely beating Dagger, an attack which landed her in the hospital.  It was later revealed that Reginald Mantz, an orderly at that same hospital, was the one behind the attack, mimicking Cloak's powers using MGH.  (Deluded, Mantz considered Dagger to be his "girlfriend.")
Wolverine discovered that Nitro had been taking MGH to boost his power, which allowed him to devastate Stamford; killing over 600 people. Said event would lead to the superhuman registration act and the superhero Civil War.
Alexander Bondt, a pre-kingpin criminal leader sent to prison by young Daredevil, consumed the drug after release from prison as means to get revenge on the superhero. In his case, the effect of the MGH was too much for his body to endure and his heart exploded.
In Uncanny X-Men #490; Endangered Species chapter 10, it was revealed that after the events of M-Day all stock of MGH that was obtained from previously-powered mutants was no longer viable. However, all of the MGH taken from still-powered mutants is fine, and the same can probably be said for non-mutant-based MGH samples.