It's been possible to increase growth hormone by injection for a couple decades now, and I thought there was a cancer problem[0].

Also, there's the drive for homeostasis. In my own experiments with dietary supplements, I get a short term boost due to the supplement, but the body gradually re-configures itself accordingly, so there's no long term benefit. e.g., caffeine[1]

[0] Growth Hormone's Links to Cancer https://pubmed.ncbi.nlm.nih.gov/30500870/

[1] Chronic Caffeine Alters the Density of Adenosine, Adrenergic, Cholinergic, GABA, and Serotonin Receptors and Calcium Channels in Mouse Brain https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3437321/

EDIT: Author has a wikipedia article and a website. https://en.wikipedia.org/wiki/Brian_Hanley_(microbiologist) https://bf-sci.com/