Side Effects of Peptides in Bodybuilding

Key takeaways

• The documented side effects of bodybuilding peptides are mostly the effects of raising growth hormone: swelling, joint and muscle pain, tingling hands and carpal tunnel, and higher blood sugar. On the growth hormone label, swelling affected 17.5% of adults versus 5.1% on placebo, and joint pain 17.3% versus 4.2%, in the first six months.
• MK-677 raised fasting glucose and lowered insulin sensitivity over two years, and a later trial in frail older adults was stopped early for a heart-failure signal.
• Insulin and IGF-1, both peptide hormones used in bodybuilding, can cause profound low blood sugar; insulin has put bodybuilders in a coma. Melanotan II has case reports of muscle breakdown with kidney injury, priapism needing surgery and melanoma.
• The needle and the vial carry their own risks: in a survey of 366 men injecting performance drugs, 42% had had redness, swelling and tenderness at a site and 6.8% an abscess or open wound; falsified peptide vials have tested at 5 to 75% purity, some with arsenic.
• Most of what forums call side effects has never been measured. Call emergency services for confusion, sweating or shaking after insulin or IGF-1, chest pain or breathlessness, or an erection lasting more than four hours.

The side effects of peptides in bodybuilding that are actually documented come from a short list of drugs: growth hormone and the peptides that raise it, MK-677, insulin, IGF-1 and melanotan II, plus the injections and unregulated vials they come in. The rest, from the 'uglies' to hair loss, is forum report. This page separates the two. It belongs to our guide to peptide stacks for muscle growth, and its companion explains whether peptides are steroids. For prescribed peptides outside bodybuilding, see the general peptide side effects reference.

What is documented, compound by compound

Sources, in row order: (Genotropin label) (Liu 2007) (Blackman 2002) (Egrifta WR label) (Nass 2008) (Adunsky 2011) (Heidet 2019) (Holt 2008) (Nelson 2012) (Hope 2015) (Janvier 2018). The CJC-1295 and ipamorelin row summarizes our profile.

Growth hormone and its secretagogues: water, joints, nerves, sugar

Every 'growth' peptide in a bodybuilding stack, from sermorelin to CJC-1295 and ipamorelin, works by raising growth hormone and IGF-1, so the growth hormone profile is the best guide to what they do. It has four parts. Fluid retention shows up as puffy hands, feet and face. Joints and muscles ache. Swelling around the wrist compresses the median nerve, producing tingling or numb fingers, often at night: carpal tunnel syndrome. And insulin sensitivity falls, so blood sugar creeps up without symptoms.

The numbers above show the pattern holds from patients with real deficiency to healthy older adults to people on growth hormone plus testosterone, where carpal tunnel symptoms reached 32% (Blackman 2002). The Endocrine Society's review of performance-enhancing drugs adds the caveat that matters for gym use: there are no systematic studies of the adverse effects of illicit growth hormone use, and the likely ones are swelling, excessive sweating, muscle and joint pain, carpal tunnel syndrome and diabetes (Pope 2014). The long-term model of growth hormone excess is acromegaly (Holt 2008), which brings enlargement of the hands and feet, insulin resistance, neuropathy and diabetes (Pope 2014); whether gym doses reproduce it has never been studied.

Secretagogues keep the pituitary's feedback loop, which should blunt these effects at physiologic doses. The one with a label shows a milder version of the same profile, including HbA1c in the diabetes range in 5% versus 1% over 26 weeks (Egrifta WR label).

MK-677: appetite, glucose and a heart signal

MK-677 is the oral secretagogue that bodybuilders take for months at a time. In its two-year trial of 25 mg daily in 65 adults aged 60 to 81, the most frequent side effects were increased appetite that faded over a few months and transient, mild leg swelling and muscle pain; fasting glucose rose about 5 mg/dL, insulin sensitivity fell, cortisol rose and weight went up 2.7 kg against 0.8 kg on placebo (Nass 2008). In 123 older adults recovering from hip fracture, the same dose was stopped early because of a congestive heart failure signal in a small number of patients (Adunsky 2011). Those were older people, not lifters; nobody has studied the young, healthy, high-calorie population that uses it most.

Insulin and IGF-1: the low-blood-sugar risk

Insulin is a peptide hormone, and some bodybuilders inject it for its anabolic effect alongside growth hormone and steroids. In an Italian study of 92 male recreational bodybuilders, 38% of those who used hormones reported insulin (Di Girolamo 2024). Both insulin and IGF-1 can cause profound hypoglycaemia (Holt 2008). One emergency case report describes a 30-year-old bodybuilder in a coma from severe low blood sugar that needed repeated glucose infusions, later traced to injections he had not disclosed (Heidet 2019). IGF-1 LR3, sold to bypass the body's IGF-binding proteins, has never been studied in a human, so its risk can only be inferred from IGF-1 itself.

Melanotan II: the tanning peptide

Melanotan II is sold in gyms for tanning and libido; it has never been approved and cannot be compounded in the United States. Its harms are known only from case reports, which give no rate but show what can happen. A 39-year-old man who injected 6 mg developed racing heart, sweating, rhabdomyolysis and kidney injury needing three days in intensive care (Nelson 2012). Another man needed surgery for ischemic priapism after an injection (Mallory 2021). A 20-year-old woman was diagnosed with melanoma three months after a course of injections combined with sunbeds (Hjuler 2014).

The needle and the vial

In a survey of 366 men injecting image- and performance-enhancing drugs, 42% had had redness, swelling and tenderness at an injection site and 6.8% an abscess or open wound; most of those with an abscess sought treatment, but only 17% of those with milder symptoms did (Hope 2015). The product adds its own risk. When a Belgian public-health laboratory analyzed the ten falsified peptide drugs it encountered most often from illegal online pharmacies, purity ranged from 5 to 75% for some peptides, and several samples carried arsenic at up to ten times the limit for injectable drugs (Janvier 2018). Our injection guide covers sterile technique; for a prescribed, labeled vial, the peptide calculator prevents the unit errors that cause accidental overdoses.

Stacked with steroids: the real-world picture

Bodybuilders rarely take peptides alone. In the Italian study, 43% of the 92 bodybuilders used hormones, and of those, 95% used anabolic steroids, often with growth hormone (30%) or insulin (38%); HDL cholesterol was lower in insulin and growth hormone users, and liver enzymes were higher in hormone users than in non-users (Di Girolamo 2024). Performance-enhancing drug use as a whole is linked to higher risk of death and to cardiovascular, psychiatric, hormonal, liver and kidney disorders, usually at supraphysiologic doses and in combination (Pope 2014). When a lifter on a stack has a problem, the peptide is one suspect among several.

What forums report that nobody has measured

These come up constantly and have no study behind them. Treat them as unverified reports, not as known rates or as reassurance.

• 'Peptide uglies': water retention, a puffy face, lethargy and tingling in the first weeks. Some of it matches the documented profile above; none of it has been measured as a syndrome. Our general side-effects page covers what to push through and what not to.
• Lethargy or sleepiness on secretagogues; hunger on ipamorelin (hunger on MK-677 is documented).
• Hair shedding: no peptide trial has reported hair outcomes.
• Gynecomastia: documented for growth hormone in older adults (Liu 2007); for peptides, unmeasured, and usually confounded by steroids.
• Heart palpitations, acne, low mood: unmeasured for peptides; well documented for the steroids they are taken with.

Who should not use them

Anyone with an active cancer or a cancer history; diabetes, prediabetes or diabetic retinopathy; heart failure; a pituitary disorder; pregnancy; and, for melanotan II, anyone with many moles or a melanoma history. Athletes subject to testing should treat all of these compounds as prohibited.

When to get care

Tell the clinician exactly what you injected, including steroids and insulin. If a growth hormone secretagogue is prescribed, the reasonable plan is IGF-1 and fasting glucose or HbA1c before starting and at 8 to 12 weeks, as our sermorelin profile describes.

Bottom line

The side effects of peptides in bodybuilding that are real and measured are the side effects of growth hormone: swelling, joints, nerves and blood sugar, plus MK-677's appetite, glucose and heart signal. The ones that can kill quickly are insulin and IGF-1, through low blood sugar, and melanotan II has a file of serious case reports. On top of that sit the needle and the vial, and the steroids most users take alongside. Everything else, from the uglies to hair loss, is unmeasured. Know the red flags, keep the vial, and tell your clinician the whole stack.

Frequently asked questions

+−What are the side effects of peptides in bodybuilding?

The documented ones come mainly from raising growth hormone: swelling of the hands and feet, joint and muscle pain, tingling fingers and carpal tunnel symptoms, and higher blood sugar. On the growth hormone label, swelling affected 17.5% of adults versus 5.1% on placebo in the first six months. MK-677 adds appetite, weight gain and lower insulin sensitivity. Insulin and IGF-1, also used by bodybuilders, can cause dangerous low blood sugar, and melanotan II has case reports of muscle breakdown, priapism and melanoma. Research vials add infection and contamination risks.

+−Are peptides safe for muscle growth?

No peptide has been shown to build muscle in a healthy adult, so any risk is taken for an unproven benefit. The prescribable ones, sermorelin and tesamorelin, have known and mostly mild side effects when monitored, with blood sugar and IGF-1 the things to check. The research peptides sold for muscle, such as CJC-1295, ipamorelin, BPC-157, TB-500 and IGF-1 LR3, have little or no human safety data and come in vials with no verified content or sterility.

+−What is the downside of peptides?

Three downsides. Raising growth hormone and IGF-1 has measured costs, including fluid retention, joint pain, carpal tunnel symptoms and higher blood sugar. Long-term effects, including cancer risk, have never been studied for any secretagogue. And most muscle peptides are sold as research chemicals, so the dose, purity and sterility of the vial are unknown, and no pharmacist or clinician is accountable for them.

+−Does MK-677 raise blood sugar?

Yes. In the two-year trial of 25 mg daily in 65 adults aged 60 to 81, fasting glucose rose by about 5 mg/dL and insulin sensitivity fell, along with increased appetite, weight gain of 2.7 kg versus 0.8 kg on placebo, and transient leg swelling and muscle pain. A later trial in older adults recovering from hip fracture was stopped early because of a heart-failure signal. Anyone with prediabetes or diabetes should treat it as a glucose-raising drug.

+−Can peptides cause cancer?

Nobody knows, because no growth hormone secretagogue trial has been large or long enough to count cancers. The concern is reasonable: IGF-1 is a growth signal, people with naturally higher IGF-1 have modestly higher rates of some cancers in observational studies, and growth hormone and tesamorelin are both contraindicated in active cancer. Anyone with a cancer history should not use a growth hormone secretagogue without their oncologist.

+−Why do my hands go numb on peptides?

Tingling or numb fingers, often worse at night, is a known effect of raising growth hormone: fluid retention can compress the median nerve at the wrist, which is carpal tunnel syndrome. On the growth hormone label, paresthesia affected 9.6% of adults versus 1.9% on placebo, and in a trial of growth hormone plus testosterone in older men, carpal tunnel symptoms affected 32%. Numbness that persists, spreads or comes with weakness needs a clinician, not a lower dose found on a forum.

+−Is melanotan II safe?

It has never been approved anywhere, cannot be compounded in the United States, and its adverse effects are known only from case reports: a man who injected 6 mg developed rhabdomyolysis and kidney injury, another needed surgery for priapism, and a 20-year-old woman was diagnosed with melanoma after a course of injections combined with sunbeds. Case reports do not give a rate, but they show the kinds of harm that occur. Anyone with many moles or a melanoma history should not use it.

+−Can peptides cause gyno or hair loss?

Growth hormone itself increased breast enlargement in men in trials of healthy older adults, so gynecomastia is a documented growth hormone effect. No peptide trial has reported hair outcomes, so hair loss on peptides is an unmeasured forum report. Many bodybuilders who report either are also taking anabolic steroids, which are a far better documented cause of both.

Sources

1. LabelPfizer. GENOTROPIN (somatropin) for injection: full prescribing information, revised July 2026 (Table 1, adult GH deficiency, double-blind phase). DailyMed, read October 10, 2026. (2026)
2. ReviewLiu H et al. Systematic review: the safety and efficacy of growth hormone in the healthy elderly. Ann Intern Med 2007;146:104-15. (2007)
3. StudyBlackman MR et al. Growth hormone and sex steroid administration in healthy aged women and men: a randomized controlled trial. JAMA 2002;288:2282-92. (2002)
4. LabelTheratechnologies. EGRIFTA WR (tesamorelin) for injection: prescribing information, revised March 2025 (sections 5 and 6; Table 1). DailyMed, read October 10, 2026. (2025)
5. StudyNass R et al. Effects of an oral ghrelin mimetic on body composition and clinical outcomes in healthy older adults: a randomized trial. Ann Intern Med 2008;149:601-11. (2008)
6. StudyAdunsky A et al. MK-0677 (ibutamoren mesylate) for the treatment of patients recovering from hip fracture: a multicenter, randomized, placebo-controlled phase IIb study. Arch Gerontol Geriatr 2011;53:183-9. (2011)
7. GuidelinePope HG Jr et al. Adverse health consequences of performance-enhancing drugs: an Endocrine Society scientific statement. Endocr Rev 2014;35:341-75. (2014)
8. ReviewHolt RI, Sönksen PH. Growth hormone, IGF-I and insulin and their abuse in sport. Br J Pharmacol 2008;154:542-56. (2008)
9. StudyHeidet M et al. Severe hypoglycemia due to cryptic insulin use in a bodybuilder. J Emerg Med 2019;56:279-81. (2019)
10. StudyNelson ME, Bryant SM, Aks SE. Melanotan II injection resulting in systemic toxicity and rhabdomyolysis. Clin Toxicol (Phila) 2012;50:1169-73. (2012)
11. StudyMallory CW, Lopategui DM, Cordon BH. Melanotan tanning injection: a rare cause of priapism. Sex Med 2021;9:100298. (2021)
12. StudyHjuler KF, Lorentzen HF. Melanoma associated with the use of melanotan-II. Dermatology 2014;228:34-6. (2014)
13. StudyHope VD et al. Injection site infections and injuries in men who inject image- and performance-enhancing drugs: prevalence, risks factors, and healthcare seeking. Epidemiol Infect 2015;143:132-40. (2015)
14. StudyJanvier S et al. Impurity profiling of the most frequently encountered falsified polypeptide drugs on the Belgian market. Talanta 2018;188:795-807. (2018)
15. StudyDi Girolamo FG et al. Metabolic consequences of anabolic steroids, insulin, and growth hormone abuse in recreational bodybuilders: implications for the World Anti-Doping Agency passport. Sports Med Open 2024;10:28. (2024)