Every technology has a logical endpoint — a destination that the science was always moving toward, even when nobody could see it clearly from the earlier stops along the way.
For testosterone replacement therapy, that destination looks increasingly like a metered-dose topical spray: precise, daily, needle-free, with a pharmacokinetic profile that mimics endogenous production and a transfer risk profile that doesn’t require patients to restructure their family life to stay compliant.
Getting there required roughly 175 years of increasingly informed trial and error. The early stops were not failures. They were the scientific process doing what the scientific process does — progressively eliminating wrong answers until the right one becomes obvious.
The Journey So Far
The empirical observation that testicles mattered — for roosters, for bulls, for men — was made long before anyone could explain it. Ancient practitioners castrated animals to change their behavior and castrated criminals as punishment, and in doing so documented, without understanding, the hormonal function of the gonad. By 3,000 years of empirical observation, the biology was roughly mapped. Nobody knew why it worked. They just knew it did.
Arnold Berthold’s 1849 experiment formally initiated the science. He transplanted rooster testes into caponized birds and watched their combs regrow, their aggression return. He did not know what testosterone was. He concluded, correctly, that the testes secreted something into the blood that acted on the rest of the body. That insight — circulating factor, not direct nerve signaling — was the seed of endocrinology.
Charles-Édouard Brown-Séquard’s 1889 self-injection of dog testicular extract attracted enormous ridicule and drove enormous research interest in roughly equal measure. The placebo effect almost certainly explained his reported rejuvenation. The scientific appetite he unleashed was real.
The actual isolation of testosterone — by Adolf Butenandt and Leopold Ruzicka, working in parallel in 1935, both ultimately receiving Nobel Prizes for the work — gave the field something to work with. Now there was a molecule. The race to formulate it began immediately.
The Wrong Roads Were Informative
Oral testosterone was the obvious first attempt. Pills are what medicine does. The liver destroyed that plan: extensive first-pass hepatic metabolism meant supraphysiological doses were needed for therapeutic blood levels, and 17-alpha-alkylated androgens achieved this at the cost of significant hepatotoxicity. Oral formulations fell out of favor for most of the 20th century.
Injections worked. Testosterone cypionate and enanthate, developed in the 1950s, produced measurable therapeutic levels. They produced supratherapeutic peaks as well — sometimes exceeding 1,200 ng/dL in the first 24 to 48 hours — and let patients fall toward the lower end of normal or below in the final days before the next shot. The rollercoaster was the delivery method. Patients who needed stable hormone levels got a biweekly wave function instead.
Subcutaneous pellets (1940s) solved the rollercoaster problem but introduced a surgical access problem. Pellet insertion requires a local anesthetic, a skin incision, and an implant procedure. Pellets can extrude. Once implanted, they cannot be rapidly adjusted if the dose turns out to be wrong.
Scrotal patches (1993) established that the transdermal route was genuinely viable at physiological doses — but required shaving the scrotum, which proved to be a compliance bottleneck that no amount of patient education fully solved.
Non-scrotal patches (1995) eliminated the shaving problem and introduced application site reactions: pruritus, blistering, and skin irritation in up to 48% of users. Roughly one in two patients, experiencing some form of skin reaction. Not acceptable for a lifetime treatment.
Gels (2000 onward) solved the skin tolerance problem. They were a genuine improvement. And then they accumulated evidence of secondary transfer — children developing signs of precocious puberty, the FDA issuing Boxed Warnings, prescribers learning to screen for household risk factors before writing gel prescriptions. A treatment that requires a household risk assessment is a treatment with a patient population problem.
Why the Spray Closes the Loop
A well-designed metered-dose transdermal spray addresses, in one format, the failure modes that accumulated across this entire history.
The first-pass problem: Like all transdermal delivery, the spray bypasses hepatic first-pass metabolism. Testosterone is delivered directly into systemic circulation through the skin capillary network.
The peak-and-trough problem: Daily application produces stable, continuous absorption — a flat pharmacokinetic profile rather than biweekly injection spikes. The Chik et al. 2006 study demonstrated bioequivalent AUC and Cmax to AndroGel 1% over the first 12 hours, confirming systemic delivery at physiological levels.
The skin tolerance problem: Volatile spray carriers evaporate rapidly, leaving minimal residue and causing minimal skin irritation. The same 2006 research documented patient-reported rapid drying and low-to-no residue, directly contrasting with patch-associated blistering.
The transfer problem: Rapid evaporation compresses the window during which transferable surface testosterone is available. The structural risk profile is meaningfully better than a gel that maintains a surface reservoir for hours.
The dose precision problem: A calibrated metered pump delivers a defined milligram dose per actuation, consistently. Men’s doses at 10 mg, 15 mg, and 20 mg. Women’s doses at 0.5 mg, 1 mg, and 2 mg. Fine-grained titration is possible without switching to entirely different products.
Where Keen Meds Fits in This Story
Keen Meds exists at an interesting junction in this history. It is a telehealth and pharmacy platform — patients access clinical care through Beluga Health, receive prescriptions for testosterone (and other hormone and metabolic compounds), and have those prescriptions filled by a compounding pharmacy — but the delivery technology it uses is not the standard compounded cream.
It is the Hypospray platform, built on the same transdermal spray technology validated in clinical research for precision hormonal delivery. Men’s testosterone at three strengths, women’s testosterone at three doses, plus estradiol, progesterone, tirzepatide, liraglutide, and BPC-157 — all through a topical spray.
The clinical proposition is coherent: the best available delivery method for testosterone therapy is the one that provides stable blood levels, avoids hepatic metabolism, minimizes transfer risk, and is easy enough to apply daily that patients actually do it. A transdermal spray, properly formulated, is that method.
That is not a marketing argument. It is the conclusion of a 90-year engineering effort to find a better way to deliver a molecule that has been medically important since Berthold’s roosters first grew their combs back in 1849.
The needle had a good 70-year run. The spray is where the science was always heading.
Next up: Series 9 is complete. Series 10 — “Where We Are Now” — takes the full scope of testosterone science and maps it onto the clinical landscape of 2025: who is on TRT, what the labs say, and where the field goes next. Post 1: The State of TRT in 2025.
