|Users|| Phil Coulson|
|TV series|| Agents of S.H.I.E.L.D.|
*End of the Beginning (mentioned)
*Turn, Turn, Turn (mentioned)
*Beginning of the End (mentioned)
GH.325 is one of the many drugs derived from the bodily fluid of the alien corpse code named "G.H.".
When an alien corpse, given the designation "G.H.", was recovered sometime after World War II it was transferred to a facility known as the Guest House where it was studied intensively. From G.H.'s bodily fluids, scientists were able to synthesize a number of advanced drugs with varying effects on human biology, including the serum GH.325. The drug was going to be used to heal a wounded Avenger.
After high-ranking S.H.I.E.L.D. agent Phil Coulson was fatally stabbed through the heart by the rogue Asgardian, Loki; S.H.I.E.L.D. director Nick Fury used his connections to allow Coulson to undergo a series of intensive surgeries at the Guest House that brought him back to life after being dead for several days. Among the processes used on Coulson was the administering of the GH.325 serum, which had the effect of completely regenerating his eviscerated heart tissue. However, the use of this serum, along with all other details of his resurrection, were kept secret from Coulson.
Later, during a mission carried out by agent Coulson's personal team, Skye, an inexperienced asset recruited by Coulson, was shot twice in the abdomen and left for dead. This event deeply affected Coulson, who felt personally responsible, motivating him to research all aspects of his mysterious resurrection in order to find something capable of saving Skye's life. His search led him and a small team to execute an unwilling raid on the Guest House in search of the drug. While the GH.325 was successful in restoring Skye's health, the raid resulted in the destruction of the Guest House, burying all existing stock of the drugs, along with its source. 
Upon learning of the miracle drug's true source, Agent Coulson became extremely apprehensive of its use and possible side effects on himself and Skye, which have yet to be seen. At his behest, Coulson had his team member and S.H.I.E.L.D. scientist Jemma Simmons actively monitor the serum's effects on Skye. Personally interested in how the GH.325 could benefit humanity, Simmons hoped to recreate the drug by repeatedly analyzing Skye's blood. However, she was unable to discover anything unusual in the blood samples, even when comparing them to Coulson's blood. When she asked Coulson to send blood samples to their Headquarters in order to analyze them and mend her lack of available resource, Coulson denied permission without explaining to Simmons the real reason for doing so.
Agent Simmons continued to research and analyze the drug in secret, eventually gaining access to a better lab at the Hub and receiving help from Agent Antoine Triplett, who used his higher Clearance Level to help contact Agent Anne Weaver for Simmons to consult on her analysis. Unfortunately, before much progress could be made Simmons was interrupted by the Assault on S.H.I.E.L.D. by HYDRA sleeper agents. 
After accessing Coulson's Team Hard Drive and researching Coulson's and Skye's blood samples, Raina was able to duplicate something she believed was similar to GH.325, though she would not guarantee its results. She ultimately used this on the fatally injured John Garrett after his cybernetic implants were destroyed by an EMP activated by Leo Fitz.
Capabilities and EffectsEdit
GH.325 is a drug known to promote cellular growth that can be used to heal critical conditions and, in some cases, even revive the recently deceased.
In humans, the drug is known to have seriously damaging effects on the patient's psyche, with re-writing the subject's memories being the only way to alleviate the effects, even then with mixed results. The mental problems may stem from the introduction of information into the subject's sub-conscious; in two of the known cases of the drug's use, the subject is shown having gained strange alien insight, manifested as compulsively writing complex alien formulas.
In Skye's case, there were no mental side-effects. The reason for Skye's apparent immunity to the drug's mental effects is unknown, but it is implied she has non-human origins.
In the three known cases of the drug's use, there was no detectable trace of the drug's presence in the patient's body immediately following the period of recovery, making it difficult to properly research the drug's exact effects or to replicate it.