The quantity of mannitol in GH vials in insufficient to provide any diuretic effect by orders of magnitude, that's a bro legend that needs to die. (For diuretic effects dosage is typically multiple grams PER kg of bodyweight)
Also, mannitol is used as an admixture in most peptide lyophilization solutions. If such were true, people would see diuretic effect from injecting just about any peptide on the market. It is the majority of the puck you see in any peptide vial.
