Before you consider a peptide: who should not, and why

An authorised medicine comes with a list of people who must not use it. Doctors call these contraindications. The list exists because the medicine was tested and the risks were written down.

Most peptides sold online have no such list. They were never tested well enough in people.^1,2,3 So we work from what each substance does in the body. If a substance pushes growth, it is a worry for someone with cancer. If it lowers blood sugar, it is a worry for someone on insulin. This page explains these situations one by one.

For GLP-1 medicines, animal studies showed harm to the unborn young. The EU label for Wegovy says not to use it in pregnancy or while breastfeeding. It says to stop at least 2 months before a planned pregnancy. The reason is that the medicine stays in the body for a long time. For Mounjaro the label says to stop at least 1 month before.^4,5

For experimental peptides there are no human pregnancy data at all. BPC-157 has one study in pregnant rats. It found no birth defects in the young rats. But it did not test fertility, or how the young grew up after birth. TB-500 and KPV have no such studies.^1,2,3

Some peptides raise growth hormone, which is a growth signal.⁷ What extra growth signals do to a body that is still growing has not been studied.

Some authorised medicines are allowed for teenagers. Wegovy, for example, is authorised from age 12, on prescription.⁴ Even for these, the long-term effects on bones, fertility and mental health in teenagers are still uncertain.⁶ For unapproved peptides, there is nothing to go on. We advise against them for anyone under 18.

Several “healing” peptides act on exactly this. In studies in cells, chick eggs and rats, BPC-157 increased blood vessel growth Animals(Evidence level: Shown in animal studies).⁸ In a study in mice, melanoma cells that made extra thymosin beta-4 spread further and grew more blood vessels Animals(Evidence level: Shown in animal studies).⁹ Thymosin beta-4 is the natural protein that TB-500 is based on. TB-500 itself was not tested in that study.² Tesamorelin, an authorised growth hormone booster, raises growth hormone and IGF-1, which are growth factors. Its label forbids its use in active cancer.⁷ Anti-doping rules class CJC-1295 and ipamorelin as growth hormone releasing factors too.¹⁰

Whether these peptides could feed a hidden or treated tumour in people has never been studied. Hypothesis(Evidence level: Proposed, not demonstrated in any experiment) Nobody can tell you the risk is low.

Tanning peptides are a separate worry for anyone who has had melanoma. In one case report, a man with past melanoma injected an alpha-MSH peptide. New, abnormal-looking moles appeared, and old moles grew darker.¹² See melanotan II.

GLP-1 type products (semaglutide, tirzepatide, retatrutide) make the pancreas release more insulin, but mainly when blood sugar is high. On their own, they rarely push blood sugar too low.⁴

Insulin and sulfonylurea tablets (such as gliclazide) work differently. They lower blood sugar whatever its level. Together with a GLP-1 type product, blood sugar can fall dangerously low. The EU labels warn about this, and a doctor may need to lower the insulin or tablet dose.^4,5,14

The Wegovy label says semaglutide is not a substitute for insulin, and that it was not studied in type 1 diabetes.⁴

Bremelanotide (PT-141) is a melanocortin peptide. It is authorised in the US as Vyleesi. Its label says it raises blood pressure for some hours after each dose. It must not be used with uncontrolled high blood pressure or known heart disease.¹⁶

Melanotan II is a close relative of bremelanotide.¹⁸ It is not an authorised medicine in the EU.¹⁷ In one case report, a man had a racing heart, muscle breakdown and kidney problems after injecting it.¹⁸

A 2025 review looked at 25 studies in people with obesity or type 2 diabetes. Eating behaviour mostly improved or stayed the same. The data were limited.²⁰

One case report describes misuse of semaglutide by a person with atypical anorexia.²¹

This is not true for every peptide. For semaglutide, kidney disease did not change the amount in the blood in a way that matters.⁴ For unapproved peptides, nobody has measured it.

GLP-1 type products often cause vomiting and diarrhoea at first. Losing a lot of fluid can, in rare cases, damage the kidneys.^5,4 People with moderate kidney disease may get more stomach and gut side effects on semaglutide. It is not recommended with severe kidney disease.⁴

• Medullary thyroid cancer or MEN 2, in you or your family: GLP-1 medicines caused thyroid tumours in rodents. Whether this happens in people is unknown. The EU label says the risk for people is thought to be low, but it cannot be ruled out. The US label forbids use in this group. In the EU it is not a formal contraindication, so discuss it with a doctor.^23,4

The INR is the blood test for warfarin-type blood thinners. A lower INR has been reported in people who took semaglutide with acenocoumarol, a blood thinner of this type. So the label advises frequent INR checks when someone on these blood thinners starts semaglutide.⁴

Most unapproved peptides are banned in sport at all times. The World Anti-Doping Agency (WADA) bans any substance with no approval for human use, and names BPC-157. It also bans growth hormone boosters such as CJC-1295, ipamorelin and tesamorelin.^10,28

Anti-doping rules use strict liability. If a banned substance is found in your body, that is a rule violation. This is true even if you did not mean to take it.^29,30

A used needle can carry a tiny amount of blood into a vial or a pen. The next person can then get HIV or hepatitis B or C, even with a new needle. Blood does not need to be visible to pass on these viruses.^31,23 Never share a pen, even if you change the needle. Never share a vial, water or other equipment.^23,4,34,35

A liquid that should be clear but is cloudy, coloured or has bits in it may be spoiled or contaminated. So may a vial with a broken seal. Do not use it.^4,7,32 See sterile technique and product risk.

Do not stop or change a prescribed medicine on your own.³³ The condition it treats can come back or get worse. If you think a peptide could replace a medicine, discuss it with your doctor first.