Peptides Explained

Warning signs: when to get help now

In short

Reading the whole page takes about 8 minutes.

  • If a life may be in danger, call your local emergency number now. Do not wait and see.
  • For other serious signs, call your GP straight away. At night, call the out-of-hours GP service.
  • Bring the vial, pen or box and its label. Tell the doctor exactly what you used.
  • Tell the doctor the whole story. They need the facts to treat you safely.
  • Dutch doctors must keep what you tell them private.
Last reviewed
A yellow Dutch ambulance with blue and red stripes and the Dutch emergency number on its side, parked outside an ambulance station.
A Dutch ambulance in the Haaglanden region. When a life is in danger, call your local emergency number. Photo: Alf van Beem (Wikimedia Commons) CC0 1.0 Image details

This page lists the signs that mean something may be going wrong after a peptide injection. It does not tell you what is causing them: a doctor can find that out with you. What you can do is notice the sign and get the right help quickly.

The three levels of help

How fast do I need to act?

It depends on the sign: now, today, or within days.

Every sign in the list below has one of three labels. Each label tells you how fast to act.

Three levels of help, from the most urgent to the least urgent
  • Call your local emergency number now. A life may be in danger. Call an ambulance. Do not drive yourself.1,5
  • Get medical help today. Call straight away, not tomorrow. Call your own GP (huisarts) during the day. In the evening, at night and at weekends, call the out-of-hours GP service (huisartsenpost). Newer Dutch health websites call it the huisartsen-spoedpost.1,7,8
  • Stop and contact a doctor. Stop using the product. Make an appointment with your GP soon, within days, not weeks.

In the Netherlands, always call the huisartsenpost before you go to a hospital emergency department, unless a life is in danger. Then you call your local emergency number. Care from your GP and the huisartsenpost does not use your deductible (eigen risico). Care in a hospital emergency department does.9

If you are not sure which level fits, call your GP. Outside office hours, call the huisartsenpost. They will tell you what to do.1,9 Calling too often is better than calling too late.

Stop-now warning signs

Call your local emergency number now

  • Confusion or slurred speech, very fast breathing, or pale, blue or blotchy skin, often with fever or shivering These can be signs of sepsis: the body's reaction to an infection has started to damage its own organs.
  • Fainting and not waking up within a minute, or fainting with chest pain, a racing heart or a fit This can be a dangerous heart rhythm or a severe allergic reaction.
  • Pain at the injection spot that is much worse than it looks, or dark, purple or grey patches or blisters there This can be a rare, fast-spreading infection of the deep layers under the skin (necrotising fasciitis).
  • Swelling of the lips, tongue or throat, trouble breathing or swallowing, wheezing, a hoarse voice, or feeling faint, soon after an injection This can be a severe allergic reaction (anaphylaxis), which is life-threatening and gets worse very quickly.
  • Pain, pressure or tightness in the chest, or pain that spreads to the arm, neck, jaw or back This can be a heart attack, which needs treatment in hospital straight away.
  • Sudden, severe shortness of breath: gasping, or not able to finish a sentence This can be a serious problem with the heart, the lungs or an allergic reaction.
  • A racing, pounding or irregular heartbeat that does not settle, or that comes with chest pain, breathlessness or fainting This can be a dangerous heart rhythm.
  • A sudden, severe headache with blurred vision, chest pain, confusion or vomiting Blood pressure may be dangerously high, which can damage the brain, heart, kidneys and eyes.
  • An erection that lasts more than 3 hours, or a painful erection that will not go down Blood trapped in the penis can damage it for good if it is not treated quickly.
  • Shaking, sweating or sudden hunger that turns into confusion or drowsiness Very low blood sugar can cause a fit or loss of consciousness; take something sugary if you can still swallow safely.
  • Severe, constant pain in the upper belly that spreads to the back, often with vomiting This can be an inflamed pancreas (pancreatitis), a known risk of GLP-1 medicines such as semaglutide and tirzepatide.
  • Sudden weakness or numbness on one side, a drooping face, or slurred or confused speech These are signs of a stroke, which needs treatment in hospital straight away, even if the signs pass.
  • A headache that starts suddenly and is extremely painful, or a headache with a stiff neck, confusion or a fit This can be a stroke or another emergency in the brain.
  • Sudden loss of sight in one or both eyes This can be a stroke or damage to the nerve of the eye, and sight can be lost for good.
  • A fit (seizure): jerking movements, or suddenly not responding A fit can be caused by very low blood sugar or a problem in the brain, and needs checks straight away.

Get medical help today

  • Fever, shivering or feeling very ill in the days after an injection Germs from the needle, the skin or the product may have reached your blood. If you also have a fast heartbeat or fast breathing, call your local emergency number.
  • Redness, heat, swelling or pain around the injection spot that keeps growing or getting worse This can be a skin infection (cellulitis), which needs antibiotics before it spreads.
  • Red lines that run from the injection spot towards the armpit or groin The infection may be spreading along the lymph vessels, and from there into the blood. If you also have a fever with a fast heartbeat or fast breathing, call your local emergency number.
  • A painful, warm lump that fills with pus This is probably an abscess: it often needs draining by a doctor, so do not squeeze it.
  • You were pricked by a needle that someone else had used Blood on a used needle can pass on HIV and hepatitis B or C, and preventive treatment works best within hours.
  • Vomiting so often that you cannot keep fluids down, or passing very little urine Losing too much fluid can damage the kidneys.
  • Yellow skin, or yellow whites of the eyes This can be a sign of a liver problem, gallstones or an inflamed pancreas.
  • Dark, red or cola-coloured urine together with muscle pain, tenderness or weakness Damaged muscle can release a protein that harms the kidneys (rhabdomyolysis).

Stop and contact a doctor

  • You fainted, but woke up quickly and now feel well Fainting is often harmless, but a doctor should check the cause.
  • A new mole, or a mole that changes size, shape or colour, itches or bleeds A changing mole can be skin cancer, and tanning peptides such as melanotan II have been linked to new and changing moles in case reports.
  • A lump at an injection spot that does not go away, or keeps growing A doctor should check whether it is an abscess, a reaction to the product or something else.
  • An itchy, raised rash, or puffy eyes or lips, while breathing stays normal Your body may be reacting to the product, and using it again can bring the reaction back.
  • Palpitations that keep coming back, even if they pass and you then feel well A doctor can check whether your heart rhythm is normal.

Why these signs matter

Why do these signs matter?

Most come from an infection, an allergy or the substance itself. Some get dangerous within hours.

Most of these signs have one of a few causes. Knowing them helps you understand why speed matters.

Infection

How an infection can spread from the injection spot to the blood
  1. A needle can carry germs from the skin, the vial or the air into the tissue under the skin. There they can grow.
  2. First the skin around the spot becomes red, hot and swollen. This is a skin infection called cellulitis. Sometimes a pocket of pus forms. This is an abscess.10,11
  3. If the infection is not treated, it can spread along the lymph vessels and into the blood. Red lines running from the spot are a sign of this.12
  4. In the blood, an infection can cause sepsis. In sepsis, the body’s reaction to the germs starts to damage its own organs. Sepsis can kill, and it can develop within hours.5,8

Allergy

The body can react to the peptide itself, to something else in the vial, or to a contamination. A mild reaction gives an itchy rash. A severe reaction, called anaphylaxis, can make the throat swell and make you faint. It usually starts within minutes and needs emergency treatment straight away.2,13 Allergic reactions have been described with some peptide medicines.14,15

Effects of the substance itself

Some peptides act on the heart, the blood vessels, the blood sugar or the gut. GLP-1 medicines such as semaglutide and tirzepatide can cause an inflamed pancreas and loss of fluid through vomiting.16,17 Tanning peptides such as melanotan II have been linked to long, painful erections, muscle breakdown and changing moles in case reports.18,19,20 In the UK, a few people were treated in hospital for very low blood sugar after using fake “weight-loss pens”. The regulator said the pens may have contained insulin instead of semaglutide.21

A product from outside a pharmacy may not contain what the label says. So a sign in this list matters even if it “should not” happen with the peptide you think you used. See product risk.

What to do while you wait

What do I do while I wait for help?

Do not drive yourself. Stay with the person, and keep the product for the doctor.
  • Do not drive yourself to a hospital. Ask someone else to drive, or call for an ambulance.5
  • Severe allergic reaction: if the person has their own adrenaline pen, use it. Let them lie down. Do not let them stand or walk, even if they feel better.2
  • Very low blood sugar: if the person can swallow safely, give something sugary, such as fruit juice or glucose tablets. If they cannot swallow or do not respond, give nothing by mouth. Put them on their side and call your local emergency number.22,23
  • Stay with the person until help arrives.
  • Keep the product. Do not throw away the vial, the pen or the packaging. A doctor may need to see it.

What to bring and what to say

What do I bring, and what do I tell the doctor?

Bring the product and its label. Tell the doctor the truth about what you used.
  • The vial, pen or box you used, with its label. Put a used vial in a closed bag or container.
  • The package leaflet or any paper that came with it.
  • What it was sold as, and where it came from (a pharmacy, a website, a gym, a friend).
  • When you last injected, and where on the body.
  • How much you used and how often. The doctor needs this to understand what happened.
  • All other medicines, supplements and drugs you take.
  • Any allergies you know of.

Tell the whole story

Tell the doctor the truth, even if it feels awkward. The tests and treatment depend on it. For example, some fake “weight-loss pens” may have contained insulin. A doctor who knows that your pen did not come from a pharmacy can check your blood sugar.21

In the Netherlands, doctors have a duty of confidentiality (medisch beroepsgeheim). A doctor must keep private everything they learn about you as a patient. There are only a few special situations in which a doctor may or must share it.24 The doctor’s job is to treat you, not to judge you.

After the emergency

What do I do when the danger has passed?

Stop the product if it may be the cause. You can also report what happened.

When the danger has passed, think about what caused it. If the product may have caused it, stop using it. Read if something goes wrong for the next steps.

You can report a side effect yourself.

Where to report a side effect yourself, by country
CountryWhere to report
NetherlandsIn the Netherlands, report it to Lareb. Anyone can report, also about medicines bought online and other health products. Lareb does not share your report with your doctor or pharmacist.25,26
United KingdomIn the UK, use the Yellow Card scheme.27
GermanyIn Germany, use the BfArM portal.28
United StatesIn the US, use FDA MedWatch.29

Reports help regulators see patterns that one doctor cannot.

Many of these problems can be prevented. Read sterile technique and injection safety. Before you start anything, read before you consider a peptide and go through the checks before you use a product.

Where this list comes from

Where does this list come from?

We used official advice for patients. Most of it comes from Dutch GPs and the NHS.

The labels follow official patient guidance. When the guidance differed between countries, we mostly chose the faster level of help. Where we kept the slower level, the sign says when to call your local emergency number instead. An example is fever after an injection: Dutch GP advice is to call the GP or huisartsenpost straight away.8,7 UK advice for a skin infection with fever is to call an ambulance.10

Group of signsGuidance we followed
Infection and sepsisNHS pages on cellulitis, skin abscess, boils, sepsis and necrotising fasciitis; MedlinePlus on lymphangitis; Thuisarts on sepsis and wondroos.10,11,30,5,31,12,8,7
Needle-stick injuryRIVM, Thuisarts and WHO.32,33,34
AllergyNHS and Thuisarts.2,13
Heart and blood vesselsNHS pages on chest pain, shortness of breath, palpitations, fainting and priapism; MedlinePlus on very high blood pressure; Thuisarts on chest pain.3,35,36,37,38,6,39
Stomach, liver, kidneys and blood sugarNHS pages on pancreatitis, low blood sugar, vomiting, dehydration and jaundice; MedlinePlus on muscle breakdown.40,22,41,42,43,44
Brain, nerves and eyesNHS pages on stroke, headaches and vision loss; Thuisarts on sudden weakness; EMA on the eye condition NAION with semaglutide.4,45,46,47,48
MolesNHS.49

This list is not complete. If something feels seriously wrong, trust that feeling and get help.5

Sources

Every claim on this page links to one of these sources. Scholarly sources link to their DOI; regulatory sources to the regulator's own page.

  1. Rijksoverheid (Government of the Netherlands) (2026). Wat moet ik doen als iemand dringend medische hulp nodig heeft?. Regulator rijksoverheid.nl — accessed 20 October 2026 ↩
  2. NHS (England) (2023). Anaphylaxis. Web page nhs.uk — accessed 20 October 2026 ↩
  3. NHS (England) (2023). Chest pain. Web page nhs.uk — accessed 20 October 2026 ↩
  4. NHS (England) (2024). Symptoms of a stroke. Web page nhs.uk — accessed 20 October 2026 ↩
  5. NHS (England) (2026). Sepsis. Web page nhs.uk — accessed 20 October 2026 ↩
  6. MedlinePlus Medical Encyclopedia (U.S. National Library of Medicine) (2025). Malignant hypertension. Web page medlineplus.gov — accessed 20 October 2026 ↩
  7. Thuisarts.nl (Nederlands Huisartsen Genootschap) (2026). Ik heb wondroos. Web page thuisarts.nl — accessed 20 October 2026 ↩
  8. Thuisarts.nl (Nederlands Huisartsen Genootschap) (2022). Ik heb een sepsis. Web page thuisarts.nl — accessed 20 October 2026 ↩
  9. Rijksoverheid (Government of the Netherlands) (2026). Wanneer moet ik naar de huisartsenpost of de spoedeisende hulppost?. Regulator rijksoverheid.nl — accessed 20 October 2026 ↩
  10. NHS (England) (2024). Cellulitis. Web page nhs.uk — accessed 20 October 2026 ↩
  11. NHS (England) (2026). Skin abscess. Web page nhs.uk — accessed 20 October 2026 ↩
  12. MedlinePlus Medical Encyclopedia (U.S. National Library of Medicine) (2025). Lymphangitis. Web page medlineplus.gov — accessed 20 October 2026 ↩
  13. Thuisarts.nl (Nederlands Huisartsen Genootschap) (2025). Ik krijg ineens dikke ogen of lippen. Web page thuisarts.nl — accessed 20 October 2026 ↩
  14. Diao L, Meibohm B (2013). Pharmacokinetics and pharmacokinetic-pharmacodynamic correlations of therapeutic peptides. Clinical Pharmacokinetics 52:855-868. Review doi:10.1007/s40262-013-0079-0 ↩
  15. U.S. Food and Drug Administration / DailyMed (National Library of Medicine) (2026). EGRIFTA SV (tesamorelin for injection): prescribing information and instructions for use. Official product information dailymed.nlm.nih.gov — accessed 20 October 2026 ↩
  16. European Medicines Agency (2026). Wegovy (semaglutide): EPAR — overview and product information. Official product information ema.europa.eu — accessed 20 October 2026 ↩
  17. European Medicines Agency (2026). Mounjaro (tirzepatide): EPAR — overview and product information. Official product information ema.europa.eu — accessed 20 October 2026 ↩
  18. Dreyer BA, Amer T, Fraser M (2019). Melanotan-induced priapism: a hard-earned tan. BMJ Case Reports 12. Research article doi:10.1136/bcr-2018-227644 ↩
  19. Nelson ME, Bryant SM, Aks SE (2012). Melanotan II injection resulting in systemic toxicity and rhabdomyolysis. Clinical Toxicology 50:1169-1173. Research article doi:10.3109/15563650.2012.740637 ↩
  20. Cardones AR, Grichnik JM (2009). alpha-Melanocyte-stimulating hormone-induced eruptive nevi. Archives of Dermatology 145:441-444. Research article doi:10.1001/archdermatol.2008.623 ↩
  21. Medicines and Healthcare products Regulatory Agency (2023). MHRA warns of unsafe fake weight loss pens. GOV.UK. Regulator gov.uk — accessed 20 October 2026 ↩
  22. NHS (England) (2023). Low blood sugar (hypoglycaemia). Web page nhs.uk — accessed 20 October 2026 ↩
  23. American Diabetes Association Professional Practice Committee for Diabetes (2026). 6. Glycemic Goals, Hypoglycemia, and Hyperglycemic Crises: Standards of Care in Diabetes—2026. Diabetes Care 49:S132-S149. Guideline doi:10.2337/dc26-S006 ↩
  24. KNMG (Royal Dutch Medical Association) (2024). Medisch beroepsgeheim: wat en waarom. Web page knmg.nl — accessed 20 October 2026 ↩
  25. Bijwerkingencentrum Lareb (2026). Netherlands Pharmacovigilance Centre Lareb — report a side effect. Web page lareb.nl — accessed 20 October 2026 ↩
  26. Bijwerkingencentrum Lareb (2026). Alles over bijwerkingen melden. Web page lareb.nl — accessed 20 October 2026 ↩
  27. Medicines and Healthcare products Regulatory Agency (2026). Yellow Card: making medicines and medical devices safer. Web page yellowcard.mhra.gov.uk — accessed 22 October 2026 ↩
  28. Bundesinstitut für Arzneimittel und Medizinprodukte (BfArM) (2026). Nebenwirkungen und weitere Arzneimittelrisiken melden. Regulator bfarm.de — accessed 22 October 2026 ↩
  29. U.S. Food and Drug Administration (2026). MedWatch: The FDA Safety Information and Adverse Event Reporting Program. Regulator fda.gov — accessed 22 October 2026 ↩
  30. NHS (England) (2023). Boils. Web page nhs.uk — accessed 20 October 2026 ↩
  31. NHS (England) (2026). Necrotising fasciitis. Web page nhs.uk — accessed 20 October 2026 ↩
  32. Rijksinstituut voor Volksgezondheid en Milieu (RIVM), Landelijk Centrum Hygiëne en Veiligheid (2026). Hygiënerichtlijn voor piercen. Regulator rivm.nl — accessed 20 October 2026 ↩
  33. Thuisarts.nl (Nederlands Huisartsen Genootschap) (2022). Ik heb onveilige seks gehad met iemand die hiv of misschien hiv heeft. Web page thuisarts.nl — accessed 20 October 2026 ↩
  34. World Health Organization (2010). WHO best practices for injections and related procedures toolkit (WHO/EHT/10.02). World Health Organization, Geneva. Report who.int — accessed 20 October 2026 ↩
  35. NHS (England) (2024). Shortness of breath. Web page nhs.uk — accessed 20 October 2026 ↩
  36. NHS (England) (2026). Heart palpitations. Web page nhs.uk — accessed 20 October 2026 ↩
  37. NHS (England) (2026). Fainting. Web page nhs.uk — accessed 20 October 2026 ↩
  38. NHS (England) (2023). Priapism (painful erections). Web page nhs.uk — accessed 20 October 2026 ↩
  39. Thuisarts.nl (Nederlands Huisartsen Genootschap) (2026). Ik heb pijn op mijn borst. Wat kan het zijn?. Web page thuisarts.nl — accessed 20 October 2026 ↩
  40. NHS (England) (2026). Acute pancreatitis. Web page nhs.uk — accessed 20 October 2026 ↩
  41. NHS (England) (2023). Diarrhoea and vomiting. Web page nhs.uk — accessed 20 October 2026 ↩
  42. NHS (England) (2026). Dehydration. Web page nhs.uk — accessed 20 October 2026 ↩
  43. NHS (England) (2024). Jaundice. Web page nhs.uk — accessed 20 October 2026 ↩
  44. MedlinePlus Medical Encyclopedia (U.S. National Library of Medicine) (2025). Rhabdomyolysis. Web page medlineplus.gov — accessed 20 October 2026 ↩
  45. NHS (England) (2024). Headaches. Web page nhs.uk — accessed 20 October 2026 ↩
  46. NHS (England) (2025). Vision loss. Web page nhs.uk — accessed 20 October 2026 ↩
  47. Thuisarts.nl (Nederlands Huisartsen Genootschap) (2026). Ik heb opeens een verlamde arm, verlamd been of scheve mond. Of ik praat, slik of zie opeens niet goed.. Web page thuisarts.nl — accessed 20 October 2026 ↩
  48. European Medicines Agency (2025). PRAC concludes eye condition NAION is a very rare side effect of semaglutide medicines Ozempic, Rybelsus and Wegovy. Regulator ema.europa.eu — accessed 20 October 2026 ↩
  49. NHS (England) (2023). Moles. Web page nhs.uk — accessed 20 October 2026 ↩