"Common" means frequent and usually manageable. "Serious" means stop and get medical help. The lists below come from approved product labels and published trials where those exist. For substances that were never properly tested in people, the honest summary is short: nobody knows the full side-effect profile, and an empty list means "not studied", not "safe".
For emergencies, go straight to the emergency page.
What to do, in general
- Mild and expected (queasiness, a little redness at the site): note it, and mention it at your next appointment.
- Persistent or getting worse: do not take the next dose until you have spoken to a clinician or pharmacist.
- Anything on a "serious" list: stop, and get medical help the same day or call emergency services.
- Never raise a dose to "push through" a side effect.
- Report it. In the US, side effects of any drug, approved or not, can be reported to the FDA's MedWatch programme on 1-800-FDA-1088 [1]. Other countries have their own schemes (Yellow Card in the UK, national agencies in the EU).
GLP-1 and GIP drugs
See /peptides/semaglutide/, /peptides/tirzepatide/ and /peptides/retatrutide/.
Common. Stomach and gut effects dominate, mostly when starting and after each dose increase. In the tirzepatide weight-management trials, at the highest dose: nausea 28%, diarrhoea 23%, vomiting 13%, constipation 11%, abdominal pain 10%, indigestion 10%, injection-site reactions 8%, fatigue 7%, belching 5%, hair loss 5%, reflux 5% [2]. The semaglutide label lists the same group, plus headache, dizziness, bloating and wind [3].
Serious (labelled warnings) [2][3]:
- Pancreatitis: severe upper abdominal pain that does not go away, sometimes spreading to the back, with or without vomiting. Stop and get urgent care.
- Gallbladder disease: pain under the right ribs, fever, yellowing skin or eyes.
- Kidney injury from dehydration: follows prolonged vomiting or diarrhoea. If you cannot keep fluids down, get help the same day.
- Severe stomach and gut reactions. The labels advise against these drugs in people with severe gastroparesis (a stomach that empties very slowly).
- Serious allergic reaction: swelling, trouble breathing, widespread rash.
- Low blood sugar, mainly in people who also use insulin or a sulfonylurea.
- Worsening of diabetic eye disease in people with type 2 diabetes: report vision changes.
- Food or liquid entering the lungs during anaesthesia or deep sedation: tell any surgical or endoscopy team that you take one of these drugs.
- Raised resting heart rate (semaglutide label): report a racing heartbeat at rest [3].
- Thyroid C-cell tumours (boxed warning, based on rodent studies; relevance to humans unknown). These drugs are contraindicated in people with a personal or family history of medullary thyroid cancer or MEN 2. Report a lump in the neck, hoarseness, or trouble swallowing.
Retatrutide is not approved anywhere. Trial reports so far show the same dose-related stomach and gut effects, plus a rise in heart rate and abnormal skin sensations; see its page for figures.
What to do. The label rule for a missed dose or a long break matters for side effects too: restarting at a high dose after weeks off brings the stomach effects back hard (see troubleshooting).
Growth-hormone secretagogues
See /peptides/tesamorelin/, /peptides/sermorelin/, /peptides/cjc-1295/, /peptides/ipamorelin/ and /peptides/ibutamoren/.
Only one drug in this class has a current approved label, tesamorelin, and only for a specific condition. Its label is the best available guide to what raising growth hormone and IGF-1 does [4].
Common (tesamorelin label, more than 5% of patients): joint pain, redness and itching at the injection site, pain in the arms or legs, swelling of the hands, feet or ankles from fluid retention, and muscle pain [4].
Serious (labelled warnings and contraindications) [4]:
- Tumour growth. Raising growth hormone and IGF-1 may promote existing cancers. Contraindicated in active malignancy.
- Raised blood sugar and diabetes. Glucose should be monitored.
- Fluid retention: swelling, joint stiffness, tingling or numbness in the hands (carpal tunnel symptoms).
- Allergic reactions, including rash and hives beyond the injection site: stop and seek prompt medical attention.
- Not for use in pregnancy, or in people whose pituitary gland has been disrupted by disease, surgery or radiation. The label also warns of increased deaths when growth hormone was given during acute critical illness.
Ibutamoren (taken by mouth; not a peptide) raises the same hormones. Trials reported increased appetite, swelling, muscle pain and higher blood sugar. A randomised trial in older patients recovering from hip fracture was stopped early because of a safety signal of congestive heart failure [5].
Sermorelin was once approved for children with growth-hormone deficiency but is no longer marketed; CJC-1295 and ipamorelin were never approved. Long-term data in adults are lacking for all three. Small studies and historical labelling describe flushing, headache and injection-site reactions. Their pages list what is known.
What to do. New swelling, breathlessness, numb or tingling hands, or unusual thirst and urination: stop and see a clinician. Anyone with a history of cancer or diabetes should not use this class without specialist supervision.
Melanocortins
See /peptides/pt-141/ and /peptides/melanotan-ii/.
Bremelanotide (PT-141) is approved in the US for one indication, as a single dose used when needed, with no more than one dose in 24 hours and no more than eight a month [6].
- Common: nausea 40% (about one in eight people needed anti-nausea medicine), flushing 20%, injection-site reactions 13%, headache 11%, vomiting 5% [6].
- Serious or notable: a temporary rise in blood pressure (up to about 6 mmHg systolic) and fall in heart rate after each dose, usually settling within 12 hours; it is contraindicated in uncontrolled high blood pressure and known cardiovascular disease. Darkening of the skin on the face, gums or breasts, which was much more frequent with daily dosing and did not always resolve after stopping [6].
Melanotan II has never been approved. A dermatology review of unregulated use describes changes in existing moles, new and atypical moles, and case reports of melanoma appearing during or shortly after use; a causal link is unproven, and several national health agencies have issued warnings [7]. Nausea, flushing and spontaneous erections are commonly reported; see its page.
What to do.
- An erection lasting four hours or more is an emergency: go to an emergency department [8].
- Any mole that changes in size, shape or colour, or any new dark patch: stop, and have a doctor examine your skin.
- Severe headache, chest pain or a pounding heartbeat after a dose: treat as urgent.
Everything else
This covers most of the site: tissue-repair peptides (/peptides/bpc-157/, /peptides/tb-500/), copper peptides (/peptides/ghk-cu/), immune peptides (/peptides/thymosin-alpha-1/, /peptides/kpv/, /peptides/ll-37/), the "bioregulator" and nootropic peptides (/peptides/epitalon/, /peptides/selank/, /peptides/semax/, /peptides/dsip/), mitochondrial peptides (/peptides/mots-c/, /peptides/ss-31/) and others. Each page states what has and has not been studied.
What they share:
- Little or no human safety data. For most there is no reliable figure for how often any side effect occurs.
- Regulators' concerns are about the product as much as the molecule. The US FDA has flagged many of these substances for possible immune reactions, peptide-related impurities, and a lack of human safety information [9]. Their regulatory status has been under active review during 2026; each peptide page carries the current status and the date it was checked.
- Injection-site reactions (redness, itching, lumps) are the most commonly reported effect for almost any injected peptide.
- Allergic reactions can happen with any injected substance, on the first dose or after many.
- Contamination effects look like side effects. Fever, chills and shaking within hours of an injection suggest a contaminated product or an infection, not a "detox" reaction. Stop and get medical help.
What to do. Treat any new symptom after starting an unstudied substance as caused by it until shown otherwise: stop, write down what you took and when, and tell a clinician exactly what it was.
Sources
Checked 13 November 2026.
- U.S. Food and Drug Administration. "MedWatch: The FDA Safety Information and Adverse Event Reporting Program." https://www.fda.gov/safety/medwatch-fda-safety-information-and-adverse-event-reporting-program
- Tirzepatide injection (Zepbound), US prescribing information, DailyMed. https://dailymed.nlm.nih.gov/dailymed/fda/fdaDrugXsl.cfm?setid=487cd7e7-434c-4925-99fa-aa80b1cc776b&type=display
- Semaglutide (Wegovy), US prescribing information, DailyMed. https://dailymed.nlm.nih.gov/dailymed/fda/fdaDrugXsl.cfm?setid=ee06186f-2aa3-4990-a760-757579d8f77b
- Tesamorelin for injection (Egrifta WR), US prescribing information, DailyMed. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=839334d3-8c1d-4c26-9036-2ab524a6ea75
- Adunsky A, Chandler J, Heyden N, et al. Randomised, placebo-controlled phase IIb trial of ibutamoren mesylate in patients recovering from hip fracture. Archives of Gerontology and Geriatrics. 2011;53(2):183–189. https://doi.org/10.1016/j.archger.2010.10.004
- Bremelanotide injection (Vyleesi), US prescribing information, DailyMed. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=f1d0c1b5-2f39-4bad-a6a4-0066e3ad5dcf
- Habbema L, Halk AB, Neumann M, Bergman W. "Risks of unregulated use of alpha-melanocyte-stimulating hormone analogues: a review." International Journal of Dermatology. 2017;56(10):975–980. https://doi.org/10.1111/ijd.13585
- American Urological Association / Sexual Medicine Society of North America. "Diagnosis and Management of Priapism: AUA/SMSNA Guideline (2022)." https://www.auanet.org/guidelines-and-quality/guidelines/diagnosis-and-management-of-priapism-aua/smsna-guideline-(2022
- U.S. Food and Drug Administration. "Certain Bulk Drug Substances for Use in Compounding that May Present Significant Safety Risks." https://www.fda.gov/drugs/human-drug-compounding/certain-bulk-drug-substances-use-compounding-may-present-significant-safety-risks