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Prescribed by UK Doctors. Dispensed by Registered Pharmacists.

Safety information

Safety, in plain English. Read this before you start.

Weight-management medicines can help, and they can also cause side effects. Here’s what’s common, what’s urgent, and what your clinician needs to know.

This website isn’t monitored. If you have urgent symptoms, don’t wait for an online reply.

999 NHS 111

When to get help straight away.

These are uncommon, but they matter. Know them before you start.

  • Swollen face, lips, tongue or throat, or difficulty breathing

    This can be a serious allergic reaction.

    Call 999

  • Severe stomach pain that won’t go away

    Especially if it spreads to your back, with or without being sick. This can be pancreatitis.

    NHS 111 or A&E

  • Sudden loss or change in your vision

    Needs same-day assessment. Tell the team about any existing eye problems.

    NHS 111 or A&E

  • Being sick or diarrhoea so bad you can’t keep fluids down

    Dehydration can harm your kidneys, so this can’t wait for a routine review.

    Get help today

  • Shaking, sweating, confusion or feeling faint

    If you also take insulin or diabetes tablets like gliclazide, this may be low blood sugar. Follow your hypo plan.

    Get help today

Common side effects, and what helps.

Most are digestive, happen after a dose increase, and ease as your body adjusts. Tell your prescriber if they don’t settle, before your next increase.

  • Feeling or being sick
  • Diarrhoea or constipation
  • Stomach pain or indigestion
  • Tiredness
  • Redness where you inject
  • Hair thinning

What often helps

  • Eat smaller meals and stop when you feel full
  • Go easy on rich, fatty or spicy food
  • Sip water through the day
  • Avoid alcohol, which can make sickness worse

Tell your clinician first if any of these apply.

  • Pregnancy or planning oneThese medicines aren’t used in pregnancy or while breastfeeding. Stop semaglutide at least 2 months, and tirzepatide at least 4 weeks, before trying.
  • Mounjaro and the pillThe pill may work less well. Use a barrier method too for 4 weeks after starting and after each dose increase.
  • Diabetes medicinesInsulin or tablets like gliclazide may need adjusting to avoid low blood sugar.
  • Surgery or sedationTell the team doing the procedure. Don’t stop your medicine on your own; your prescriber will plan it.
  • Past pancreatitis or gallbladder problemsThese need a careful review before any treatment is considered.
  • Eye disease or an eating disorderDiabetic eye disease may need closer checks. Eating disorders need a sensitive conversation first.

If you’re feeling low, talk to your GP. If you’re in crisis or have thoughts of harming yourself, call 999 or Samaritans free on 116 123.

Only buy through a regulated route.

Fake weight-loss pens are a real and growing risk in the UK, including through websites that look legitimate. Some have contained insulin.

  • Never buy from social media, salons or sellers who don’t ask about your health.
  • Never use a “mix it yourself” powder kit.
  • Check any online pharmacy on the GPhC register.

A safe online service will

  1. Asks about your health and your medicines
  2. Checks your identity
  3. Confirms your height and weight on a video call
  4. Checks your history with your GP or records
  5. Is prepared to say no if it isn’t safe

Reporting, stopping and storing

Report a problem

Suspected side effect, or a medicine that doesn’t seem genuine? Anyone can report it on the MHRA’s Yellow Card scheme.

Before you stop

Talk to your prescriber first. Weight often returns after stopping, and diabetes medicines may need changing too.

Storing and disposing

Keep pens in the fridge as the leaflet says, and put used needles in a sharps bin, never the household rubbish.

Still have questions? We’ll answer them honestly.

Read the questions people ask most, or start the pre-screen when you’re ready. A clinician reviews everything before any treatment decision.

Read the FAQsStart the pre-screen
Sources for this page · reviewed September 2026

General information, not a substitute for your medicine’s patient leaflet or advice from your prescriber.