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Over 10-15 years ago there were several progressive doctors in the cash HRT/TRT (hormone replacement therapy/testosterone replacement therapy) space who at some point realized subcutaneous administration of testosterone cypionate or enanthate not only absorbed as effectively as IM (intramuscular injections). They also found that more frequent subcutaneous (SC) injections yielded more stable levels than once per week IM injection. Some of these doctors started writing about it including TRT expert Dr. Eugene Shippen who wrote the book “The Testosterone Syndrome.” These doctors may have also found old studies (or did small observational studies themselves) showing that oil-based hormone injections can be administered SC with efficacy. Over the years as TRT became more prevalent, experts and physicians have attempted to develop novel ways to administer testosterone. This lead to more formal studies being done on the effects of subcutaneous testosterone injections. In addition to formal studies, many progressive doctors also experimented and observed that SC delivery is equal if not superior (in pharmacokinetics) over single larger dosed IM injections. Subcutaneous testosterone injections may also present lower Cmax levels of testosterone (peaks), which could translate into fewer problems with increased blood viscosity, blood pressure and cardiovascular risks related to increased red blood cell volume (hematocrit). Subcutaneous testosterone injections have been found effective by the FDA. They approved Xyosted for that purpose. Selection of studies to review: A 52-Week Study of Dose-Adjusted Subcutaneous Testosterone Enanthate in Oil Self-Administered via Disposable Auto-injector. – PubMed – NCBI Serum Testosterone Concentrations Remain Stable Between Injections in Patients Receiving Subcutaneous Testosterone. – PubMed – NCBI Subcutaneous Testosterone: An Effective Delivery Mechanism for Masculinizing Young Transgender Men. – PubMed – NCBI Pharmacokinetics, safety, and patient acceptability of subcutaneous versus intramuscular testosterone injection for gender-affirming therapy: A pilot… – PubMed – NCBI Daily subcutaneous testosterone for management of testosterone deficiency. – PubMed – NCBI Subcutaneous administration of testosterone. A pilot study report. Al-Futaisi AM, Al-Zakwani IS, Almahrezi AM, Morris D. Source Department of Medicine, College of Medicine & Health Sciences, Sultan Qaboos University, Muscat, Oman.