GLP-1 overdose: why it happens and how to stay safe

You've just injected your weekly dose, the pen didn't click the way you expected, and you're wondering whether to inject again. Or a friend bought a pen online and isn't sure how much to draw up.

Those are exactly the moments when GLP-1 overdoses happen. An overdose means injecting more medication than prescribed, most often through a dosing mistake rather than anything wrong with the medicine itself. It usually causes severe nausea, vomiting and dehydration, and low blood sugar (hypoglycaemia) is reported in up to 9% of cases, even in people without diabetes. The vast majority recover with supportive care.

If you're considering treatment, or already on it and want a doctor to oversee it, our doctor-led weight management service at 117a Harley Street is £650 for the clinical care. Pens are paid for separately at a pharmacy, from about £200 each. If you're worried about a dose you've already taken, go straight to what to do if you think you've taken too much.

How common are GLP-1 overdoses?

In the UK, the MHRA has warned that GLP-1 medicines should only come from a legitimate pharmacy, including online pharmacies, with a prescription from a healthcare professional. Selling them through beauty salons or social media is illegal. The MHRA asks anyone with a suspected side effect to report it through the Yellow Card scheme.

The UK hasn't published national overdose figures. The clearest numbers come from the US, where calls to poison centres about GLP-1 overdoses have risen by nearly 1,500% since 2019, with 3,633 exposure cases reported in just four months of 2025. The pattern behind those numbers comes down to how the medicines reach patients, and the same conditions exist here.

This article is about overdose risk. Used at the prescribed dose with proper supervision, the medicine has a well-characterised safety profile.

What causes most GLP-1 overdoses?

Three things, and none of them is the medicine itself.

Self-administration errors. Research cited by the American Gastroenterological Association found that 91% of GLP-1 exposures reported to poison centres came from patient mistakes, and 21% of those happened on the very first use of the pen. People inject again because they think the pen has failed, draw the wrong amount from a multi-dose vial, mix up weekly and daily dosing, or inject a whole multi-dose pen at once.

Compounded and unregulated product. In the US, much of the rise has been linked to compounded semaglutide sold through telehealth providers, often in vials rather than pre-filled pens. One published case described a patient who injected 2 mg instead of the prescribed 0.1 mg; another injected 2.4 mg instead of 0.25 mg, both at the start of treatment. In the UK, the equivalent risk sits with the grey market in unlicensed products sold direct to consumers.

No one watching the start. People who buy through an online tick-box consultation receive a prescription, a pen and a leaflet. Nobody watches them inject, checks they understand the schedule, or checks their blood sugar if symptoms appear. Specialists quoted in the US data name the start of treatment as the highest-risk moment, which is when most online services have already stepped back.

How does an overdose differ from normal side effects?

Overdose symptoms overlap with the usual side effects. The difference is how severe they are and how long they last.

Typical side effects compared with an overdose

Low blood sugar is the risk people underestimate. It's reported in up to 9% of overdose cases even without insulin or a sulfonylurea, and it's more common in lean people and people without diabetes. If you're taking a GLP-1 for weight loss, you're unlikely to be checking your blood sugar, and that blind spot is part of why these cases reach hospital.

Mixing the medicine with alcohol or cannabis is linked to worse outcomes. Two habits borrowed from bodybuilding add risk too: "stacking", taking more than one GLP-1 at a time, and "cycling", deliberately varying the dose for faster weight loss. Neither sits within any prescribing guideline.

What does this look like in the UK?

Online weight loss services have multiplied. Many work on a written questionnaire with no in-person assessment, no baseline blood tests and no follow-up beyond automatic repeat prescriptions.

The MHRA has warned about illegal GLP-1 products sold online, and the Advertising Standards Authority has acted against operators promoting prescription-only weight loss medicines outside the rules. We're not naming individual operators. The point is structural: a questionnaire and a parcel can't do what a clinical assessment does, and the errors in the US data are exactly the ones supervision is designed to catch.

What does in-person supervision change?

Five things, each aimed at a point where the US cases went wrong.

• Choosing the right patients. A high BMI on its own doesn't make someone a good candidate. A private GP consultation with a proper history picks up what needs care first, such as past pancreatitis, severe gastrointestinal disease, type 1 diabetes or pregnancy plans.
• Baseline blood tests. A panel covering full blood count, kidney and liver function, lipids and HbA1c screens for problems and gives a baseline for judging side effects later. We take these through our on-site blood tests service.
• Injection technique. The first injection is the highest-risk dose a patient will take. Watching it, with a nurse correcting technique in real time, is the single most useful step against self-administration errors.
• Reviews at weeks 4, 8, 12 and 16. A clinician checks weight, side effects, blood pressure and how well you're tolerating the dose. This is where a dose is held or reduced, rather than stepped up before you're ready.
• Direct access between visits. Many overdose cases that reach hospital involve people who didn't know who to call. Knowing your prescribing doctor's name and number closes that gap.

How our doctor-led weight management works

Our Harley Street clinic has been at 117a Harley Street since 1984 and is CQC-registered. Our in-person weight management service prescribes Mounjaro, whose active ingredient is tirzepatide, and it's built around the failure points above. It's deliberately slower and more thorough than the online model, because the evidence on where things go wrong is too consistent to ignore.

What happens, step by step

The clinical package is £650 and covers the consultation, body composition scan, blood tests, structured reviews and direct access to your doctor. Pens are paid for separately at a pharmacy, from about £200 per pen depending on dose and pharmacy, and every later pen is also charged on top of the £650. We prescribe the licensed Mounjaro pen only and never supply compounded GLP-1 products.

If you'd like to understand how the doctor decides whether treatment suits you, our guide to weight loss injection suitability walks through it.

Already on treatment with another provider?

If you're part-way through treatment elsewhere, these are good reasons to ask for a second opinion:

• you can't name the doctor who prescribed your medicine
• you started without an in-person assessment or baseline blood tests
• you have no follow-up dates booked with a named clinician
• you reported a side effect and received a templated reply rather than a clinical review
• your dose is going up on a fixed schedule without anyone checking how you managed the last one

We regularly see people who started elsewhere and want closer oversight as their dose rises. We carry out the same assessment, blood panel and body composition scan as for a new patient, then continue treatment under direct clinical supervision if the doctor agrees it's appropriate.

Who needs extra care before starting?

We follow the UK Summary of Product Characteristics (SmPC) and NICE guidance. Tirzepatide is licensed for adults, and its only formal contraindication is an allergy to the medicine or its ingredients. Your doctor will still look carefully at:

• type 1 diabetes
• pregnancy, breastfeeding or plans to conceive
• a history of pancreatitis
• severe gastrointestinal disease, including gastroparesis (delayed stomach emptying) and inflammatory bowel disease
• severe heart failure, or severe kidney or liver disease

For private treatment, a BMI of 30 or above is the usual threshold, or 27 or above with at least one weight-related condition such as type 2 diabetes, high blood pressure or obstructive sleep apnoea. On the NHS, NICE TA1026 recommends tirzepatide for adults with a BMI of at least 35 and at least one weight-related condition, with a threshold 2.5 lower for people from South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds. Your doctor will confirm what applies to you.

Contraception and pregnancy

GLP-1 medicines shouldn't be used in pregnancy. The UK SmPC advises stopping tirzepatide at least one month before a planned pregnancy; for semaglutide, the MHRA advises at least two months.

The MHRA's 2025 advice adds a specific point for tirzepatide. If you take an oral contraceptive pill, use a barrier method such as condoms as well for four weeks after starting, and for four weeks after every dose increase, or consider switching to a non-oral method such as a coil or implant. Vomiting and diarrhoea can also stop the pill being absorbed properly.

What to do if you think you've taken too much

First, this is recoverable. The vast majority of cases settle with supportive care. If you've injected more than your prescribed dose, or aren't sure how much you injected, the safe steps are the same wherever your medicine came from.

1. Hold any further doses until you've spoken to a clinician.
2. Drink fluids and rest.
3. If you develop severe vomiting, constant abdominal pain, signs of dehydration, or signs of low blood sugar (sweating, shakiness, confusion), go to A&E.
4. NHS 111 can advise. The National Poisons Information Service (NPIS) is a resource for clinicians rather than a public helpline, so patients should use 111 or A&E.
5. If you're an MEC patient, contact your prescribing doctor directly on the number given at your first consultation. We'd rather hear from you straight away.

If you're not under our care, use NHS 111 or A&E as above, even if you can't reach the service that prescribed for you. Afterwards, you can report what happened through the MHRA Yellow Card scheme.