Weight Loss Injection Complications: What to Know (October 2026)

01 · Start here

What complications should you expect with weight loss jabs?

Expect mild, short-lived effects first. The 2.5mg starting weeks are about adjustment, and mild nausea, altered appetite and tiredness in the first days after each injection are the commonest early effects; they usually settle.¹

The starting dose is chosen for tolerability rather than weight loss, and the dose only steps up after at least 4 weeks.¹ Many people choose a weekend injection day so any early effects fall on days off.

For the full list of listed side effects, read the patient information leaflet that comes with every pen.¹³ Our Mounjaro complications guide goes deeper on specific problems.

02 · Quick reference

Common problems and what to do

03 · Getting it safely

How to reduce the risk when you buy

The biggest avoidable risk is supply from someone who skips the consultation. A seller offering to supply without a consultation or prescription is a warning sign, and the MHRA publishes counterfeit medicine alerts.⁷ You can check any pharmacy on the GPhC register; ours is 9012878.

We supply genuine KwikPens through official LillyDirect supply, with a free consultation reviewed by a GPhC-registered prescriber. Ongoing monthly reviews and side-effect support are included.

Related guides: Weight Loss Injections Register Online and Mounjaro Weight Loss Injections UK Price: How Does It Work?.

Related guide: Do weight loss injections workhealth.

04 · Dosing errors

Dosing errors that cause avoidable complications

Most dosing problems are preventable by following the licensed schedule. Everyone starts on 2.5mg once a week, steps to 5mg after at least 4 weeks, then rises by 2.5mg at minimum 4-week intervals up to 15mg, depending on response and tolerance.¹ Higher is not automatically better, and many people settle at 5mg, 10mg or 15mg.

05 · Storage and injecting

Storage and injection technique

Keep pens in a fridge at 2–8°C and never freeze them. Once in use, a pen can be kept out of the fridge at a temperature not above 30°C for up to 30 days after first use, and must then be thrown away. Do not use a pen that has been above 30°C or has been frozen. Keep the cap on to protect it from light.¹

Unopened pens keep in the fridge until the expiry date printed on the box. If an unopened pen has been out of the fridge, check with the pharmacy before using it rather than relying on a day limit you saw online.¹³

Inject under the skin once a week, on the same day, at any time of day, with or without food. Use the stomach area (at least 5cm from the belly button), the front of the thigh, or the back of the upper arm if someone else injects. Rotate the site each dose.¹ You can change your injection day as long as at least 3 days have passed since your last dose.¹

06 · Interactions and stopping

Other medicines and stopping treatment

Tell your prescriber about every medicine you take. Mounjaro slows stomach emptying, which can reduce how well oral contraceptive pills are absorbed.⁷ The SmPC advises switching to a non-oral method, or adding a barrier method (such as condoms), for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase.¹ If you use insulin or sulfonylureas, doses may need adjusting because of the risk of low blood sugar.¹

Tirzepatide has a half-life of about five days, so after a final dose it takes roughly 4–5 weeks to clear the system. Nothing needs doing while it clears, and there is no way to speed it up.¹ Mounjaro manages weight; it does not cure the biology behind it. In SURMOUNT-4, people who stopped tirzepatide after 36 weeks regained on average 14% of body weight over the following year, while those who continued treatment lost a further 5.5% on average. Stopping should always be planned with your prescriber, with diet and activity habits ready to carry the load.

07 · Who should be prescribed

Who the jab is licensed for

Eligibility screening is itself a safety step. Mounjaro is licensed for adults with a BMI of 30 or above, or 27 to 30 with at least one weight-related condition, used alongside a reduced-calorie diet and increased physical activity.¹ A prescriber assesses your BMI, medical history and current medicines first.

Mounjaro is also available on the NHS, but access is phased in over several years. NICE recommends it for adults with a BMI of at least 35 plus a weight-related condition, and the primary-care rollout began in June 2025 with the highest-need groups.⁴⁶ See our page on government weight loss injections for more, or compare all weight loss treatments.

08 · Reporting problems

When to speak to someone and how to report

Contact your prescriber about any symptom that worries you, does not settle, or is new. A symptom that is not on the patient leaflet's list is not a listed side effect, but new or lasting symptoms should still be discussed with your prescriber.

You can report suspected side effects yourself through the MHRA Yellow Card scheme.¹⁹ Our support team is available Monday–Friday 9am–5pm on 0333 207 0413 or support@everydaymeds.co.uk.

09 · Serious side effects

Serious side effects: when to get help straight away

Serious side effects are uncommon but need urgent attention. Seek immediate medical help for: severe, persistent stomach pain that may spread to your back, with or without vomiting (possible pancreatitis) — stop using Mounjaro and get medical help straight away; pain in the upper right abdomen, fever or yellowing of skin/eyes (possible gallbladder problems) — get medical help straight away; or signs of a severe allergic reaction such as swelling of the face or throat and difficulty breathing — get medical help straight away. If you take insulin or a sulfonylurea alongside Mounjaro, watch for low blood sugar (shakiness, sweating, confusion) — your prescriber may need to adjust those medicines.¹