A peptide is defined as a small protein consisting of short chains of amino acids, strictly two to 50 amino acids, though peptides of 75-100 amino acids are sometimes still considered peptides; combining peptides yields polypeptides, and the order of amino acids along the chain determines what the peptide is and does.

Lipopolysaccharide (LPS) can accumulate during peptide manufacturing and causes an immune response; while a tiny amount may not cause a massive response, repeated exposures accumulate and become problematic, which is why properly cleaned (LPS-removed) peptides from pharma or compounding pharmacies are safer than gray/black-market sources.

Applied therapeutic peptides fall into three categories: prescription FDA-approved peptides from board-certified physicians (the safest), gray-market peptides purchasable online with questionable safety/efficacy, and black-market peptides which are often contaminated and not what they claim to be; the safest path is prescription peptides because they are cleaned of lipopolysaccharide (LPS).

Taking growth hormone directly carries common risks including carpal tunnel syndrome, cartilage overgrowth, structural changes to the head, face and body (brow thickening), a lean-but-distended gut, and unusual skin texture; growth-hormone-promoting peptides produce less dramatic structural changes than direct growth hormone.

Growth hormone secretagogue peptides should be taken about 20-30 minutes before sleep, at least an hour and a half (preferably two hours) after eating and with no food for at least 30 minutes afterward, because elevated blood sugar or food in the gut blunts their growth-hormone- and IGF-1-raising effect, and bedtime timing augments the natural early-night growth hormone pulse.

People tend to wrongly assume peptides are inherently safe or free of side effects because they are not hormone therapies like testosterone or estrogen therapy; in fact peptides are very potent compounds with many pleiotropic effects, peptide therapeutics are in early stages, and they carry real risks requiring physician oversight, clean sourcing, regular blood testing, and tumor monitoring.

BPC-157 has a very high safety profile in terms of lethal dosing, with an LD50 as high as 2 grams (2,000 mg) per kilogram of body weight; typical therapeutic doses are far lower, around 300-500 micrograms subcutaneously two to three times per week for about 8 weeks, followed by cycling off for 8-10 weeks.

The melanotan peptides (1 through 5) all mimic melanocyte-stimulating hormone and cause skin darkening, but melanotan 1 does not cross the blood-brain barrier (only tanning), whereas melanotan 2-5 cross it and additionally increase mood and libido and decrease appetite; PT-141 (the FDA-approved drug Vyleesi) targets this system and is approved for premenopausal hypoactive sexual desire in women but is used off-label in men.

Type-two secretagogues like GHRP-3 and MK-677 (an oral, non-injectable GHRP) can potently increase growth hormone but also more than double circulating cortisol and elevate prolactin; because cortisol should be high in the morning and very low at night, and these peptides are taken before bed, the cortisol elevation can be problematic.

BPC-157 poses a potential tumor growth risk because it upregulates growth hormone receptors and increases VEGF (vascular endothelial growth factor) to drive angiogenesis; tumors thrive on increased blood flow and growth factors, so BPC-157 may maintain or accelerate tumor growth — notably it does the exact opposite of the cancer drug Avastin, which is a VEGF inhibitor.

Kisspeptin is a recently discovered peptide made in the brain that sits at the top of the reproductive hormone cascade — it turns on GnRH, which triggers LH and FSH release from the pituitary, which in turn stimulate the gonads to produce testosterone and estrogen and drive sperm/follicle development; synthetic kisspeptin is prescribed for hypothalamic amenorrhea and used off-label to boost vitality and libido, while kisspeptin antagonists treat menopausal vasomotor symptoms.

Hexarelin is the strongest stimulator of growth hormone release (pulses 2-3x larger than other secretagogues) but can dramatically increase prolactin (suppressing libido), cause fluid retention and malaise, and most problematically desensitize the growth-hormone-releasing-hormone receptors, potentially permanently turning off the system's response to any peptide or endogenous signal.

The melanocortin system links light exposure (UVB sunlight to eyes or skin) through the hypothalamus and pituitary to release of melanocyte-stimulating hormone, causing skin pigmentation while in parallel stimulating dopamine release; this conserved system explains why people and animals feel better, have more energy/libido, and (in animals) darken pelage and breed when sunlight increases in spring.

Thymosin beta 4 (and its truncated synthetic version TB-500) is a peptide naturally produced by the thymus, which is present in children and disappears with age; it promotes tissue repair by stimulating growth of multiple cell types, stem cell proliferation, and extracellular matrix growth, but unlike BPC-157 it is involved in tissue repair, not tissue growth, and does not impact the growth hormone pathway.

Most peptides, especially therapeutic peptides, have pleiotropic effects, meaning a single peptide activates many different cellular pathways and affects many cell types and organ systems depending on cell type, time of day, and downstream effects, so there is no way to obtain a targeted single-pathway effect without activating other pathways.

BPC-157 is a synthetic peptide manufactured to resemble body protection compound, a peptide that naturally occurs in the gut and is involved in wound healing and repair; its presence in the gut is plausibly linked to the high cellular turnover of gut tissue, which mirrors the cellular turnover involved in wound healing.

BPC-157 promotes tissue repair primarily by increasing blood flow to injury sites through angiogenesis: it recognizes injured blood vessels and capillaries and upregulates the enzyme eNOS (endothelial nitric oxide synthase), causing new vasculature to form, which delivers growth factors that further rejuvenate tissue; it also encourages fibroblast migration and growth at the injury site.

Despite a large peer-reviewed animal literature dating back to about 1993, there is essentially no human clinical trial data on BPC-157 (only one poor self-report study), so it is impossible to rule out that its widely reported benefits are robust placebo effects, even though anecdotal reports from hundreds of thousands to millions of users are unusually strong.

Direct growth hormone injection is subject to negative feedback, where high blood levels cause the pituitary and brain to shut down endogenous growth hormone production; this drove the development of secretagogue peptides that stimulate the body's own growth hormone release by mimicking hypothalamic signals rather than directly supplying growth hormone.

Growth hormone secretagogue peptides fall into two categories: 'type one' GHRH-mimicking peptides (sermorelin, tesamorelin, CJC-1295) which are mostly human-tested and in several cases FDA-approved, and 'type two' growth-hormone-releasing peptides (ipamorelin, hexarelin, GHRP-2/3/6, MK-677) which act by mimicking or stimulating ghrelin and thus also increase hunger and anxiety.