"Peptides and testosterone" is usually searched in the hope of a peptide that raises it. The documented record runs the other way: the peptides with the largest, best-measured effects on testosterone are the ones that shut it down — approved prostate cancer drugs that take men to castration levels in days or weeks. The raising side has one approved product, hCG, and a set of small physiological studies of kisspeptin. Growth-hormone peptides, which are often sold beside testosterone, were measured for something else.
This page sorts the peptides that act on the testosterone axis by direction and by the size of the measured effect, with the source and date for each. It does not cover testosterone itself or non-peptide drugs except where one is the comparison. Nothing here is a recommendation; any decision about hormone treatment belongs with a prescriber.
How a peptide can reach testosterone at all
Testosterone in men is made by the testis on instruction from a chain of three signals: kisspeptin in the brain triggers GnRH, GnRH tells the pituitary to release LH, and LH tells the testis's Leydig cells to make testosterone. Every peptide on this page works on one link of that chain:
| Link | Peptides that act there | Direction in the record |
|---|---|---|
| Kisspeptin (top) | kisspeptin-10, kisspeptin-54, analogues | Up when pulsed; down when continuous |
| GnRH (middle) | gonadorelin (pulsed); leuprolide, triptorelin (continuous agonists); degarelix (blocker) | Pulsed: up; continuous or blocked: down to castration |
| LH at the testis (bottom) | hCG | Up — it substitutes for LH |
The one rule that explains most of the table: the axis responds to pulses, not to constant pressure. A brief or pulsed signal raises LH and testosterone; a constant one makes the pituitary stop answering. That is why the same class of molecule appears as both a fertility treatment and a cancer drug.
The peptides that lower testosterone — and by how much
Degarelix (Firmagon) blocks the GnRH receptor outright, so there is no initial surge. The pivotal trial on its US label (DailyMed, Firmagon, label version February 2024, read 17 October 2026) compared it with leuprolide in men with prostate cancer whose testosterone started around 400 ng/dL:
| Share of men at testosterone ≤50 ng/dL | Degarelix 240/80 mg (n=207) | Leuprolide 7.5 mg (n=201) |
|---|---|---|
| Day 1 | 52% | 0% |
| Day 3 | 96% | 0% |
| Day 7 | 99% | 1% |
| Day 14 | 99% | 18% |
| Day 28 | 100% | 100% |
| Kept there, day 28 to day 364 | 97.2% | 96.4% |
Leuprolide (Lupron Depot) is a GnRH agonist given continuously from a depot. Its label (DailyMed, Lupron Depot, June 2026 version) states that in most patients "testosterone levels increased above baseline, and then declined thereafter to castrate levels (< 50 ng/dL) within four weeks", and that castrate levels have been maintained in prostate cancer patients "for more than five years". That first-week rise is why leuprolide shows only 1% of men suppressed at day 7 in the degarelix table.
These are the largest documented testosterone changes any peptide produces: from roughly 400 ng/dL to under 50 — a fall of more than 87%, our arithmetic from the label's baseline — sustained for a year or more. They are prescribed precisely because they do this, and they are the reason "peptide" and "testosterone" belong in the same sentence in a medical record.
The peptide approved to raise it: hCG
hCG works at the bottom of the chain. Novarel's US label (DailyMed, September 2025 version, read 17 October 2026) describes its action as stimulating "the interstitial cells (Leydig cells) of the testis to produce androgens", and lists among its indications "selected cases of hypogonadotropic hypogonadism (hypogonadism secondary to a pituitary deficiency) in males". It works where the testis is intact and the pituitary signal is missing; it cannot help a testis that has failed.
The same label carries a warning in capitals that is worth repeating because hCG is also sold for weight loss: "HCG HAS NOT BEEN DEMONSTRATED TO BE EFFECTIVE ADJUNCTIVE THERAPY IN THE TREATMENT OF OBESITY." Its contraindications include prostate cancer and other androgen-dependent tumours. How hCG compares with the pituitary-level options is on our gonadorelin vs hCG page and our hCG vs kisspeptin page; reported adverse effects are on our hCG side effects page.
Kisspeptin: a real rise, briefly, in small studies
Kisspeptin acts at the top of the chain. The first human study (Dhillo et al., J Clin Endocrinol Metab 2005, PMID 16174713) was a double-blind, placebo-controlled crossover in six men, each given a 90-minute intravenous infusion of kisspeptin-54 and one of saline:
| Measure | Saline | Kisspeptin-54 | Change |
|---|---|---|---|
| LH, mean over 90 min (U/L) | 4.2 | 10.8 | ×2.6 |
| FSH, mean over 90 min (U/L) | 3.2 | 3.9 | +22% |
| Testosterone, mean over 180 min (nmol/L) | 21.7 | 24.9 | +15% (≈626 → 718 ng/dL, our conversion) |
LH more than doubled; testosterone rose by about a seventh, in men whose levels were already normal, for the three hours measured. In four men with type 2 diabetes and low testosterone, an 11-hour infusion of kisspeptin-10 took testosterone from about 245 to 329 ng/dL (George et al. 2013, as recorded on our kisspeptin vs enclomiphene page).
The effect does not survive constant use. In women given kisspeptin-54 twice daily, the LH response fell from 24.0 IU/L on day 1 to 2.5 IU/L on day 14; a continuously infused kisspeptin analogue, TAK-448, pushed healthy men's testosterone into the castration range by day 8 (both as recorded on that page). Kisspeptin behaves like GnRH: pulsed, it raises the axis; held on, it switches it off. No kisspeptin product is approved anywhere. Reported adverse effects are on our kisspeptin side effects page.
Gonadorelin: a pituitary signal with a diagnostic label
Gonadorelin is synthetic GnRH. Given in pulses by pump, it can drive the whole axis — in one trial in adolescent boys with hypogonadotropic hypogonadism, pulsed GnRH and hCG produced similar testosterone over 12 to 14 months, with larger testes on GnRH (Gong 2015, PMID 25978110). That is the use it was developed for. The clinic practice of giving gonadorelin by occasional injection to men on testosterone therapy has no published trial: a PubMed search on 27 September 2026 found none (both as recorded on our gonadorelin vs hCG page).
Growth-hormone peptides: measured for something else
CJC-1295, sermorelin, tesamorelin and ipamorelin act on the growth-hormone axis, a separate chain from the pituitary. The two 2006 CJC-1295 trials (PMID 16352683) listed growth hormone and IGF-1 as their outcome measures; testosterone was not one of them. A claim that one of these peptides "boosts testosterone" should come with a study that measured it. What they did measure is on our CJC-1295 results and sermorelin results pages.
What the record does not contain
- Any approved peptide for raising testosterone in men with ordinary age-related decline; hCG's approval is for a pituitary cause.
- Any trial of repeated kisspeptin injections raising testosterone in men over weeks.
- Any published study of gonadorelin given alongside testosterone therapy.
- Any trial showing a growth-hormone peptide changes testosterone.
Which of these compounds are banned in tested sport is covered by MuscleLedger's kisspeptin page. What each molecule is lives in Peptide Lexicon's entries for kisspeptin and hCG.
Sources and dates
Read 17 October 2026: DailyMed SPL documents for Firmagon (setid ab11dd8a…, February 2024), Lupron Depot (setid cbc8f94e…, June 2026) and Novarel (setid ac6918ff…, September 2025), clinical studies, pharmacology and indications sections; PubMed abstracts for Dhillo et al. 2005 (PMID 16174713) and Teichman et al. 2006 (PMID 16352683). Figures marked "as recorded" come from our own pages, with the dates and sources given there. Unit conversion: 1 nmol/L testosterone = 28.84 ng/dL.