The Side Effects of Prescription Weight Loss Drugs May Include… (October 2026)

01 · The main effects

What are the main side effects of weight-loss drugs?

The main side effects are digestive. Nausea, diarrhoea, vomiting, constipation and abdominal discomfort affect more than 1 in 10 people. Indigestion, reduced appetite, dizziness, reflux, burping, hair shedding and injection-site reactions are common, affecting up to 1 in 10.¹

Most follow a pattern: they appear in the first days after starting or increasing a dose, peak early and settle as your body adapts. This is why Mounjaro uses a gradual dose ladder.¹³ If a dose is hard to tolerate, prescribers can hold the current dose longer before stepping up.

These are weight-management trial figures; nausea, vomiting and diarrhoea were more common while the dose was being increased and became less frequent over time.¹

02 · Next steps

Is this relevant to me, and what do I do next?

It is relevant if you are considering treatment, because every licensed option has trade-offs. Mounjaro is licensed for adults with a BMI of 30 or above, or 27 to 30 with a weight-related condition, alongside a reduced-calorie diet and more activity.¹ A prescriber assesses your medical history and current medicines first.

If you want to go ahead, you can start your free consultation. Our consultation is free, and the prescribing team provides ongoing monthly reviews and side-effect support.

Related guide: Buy Mounjaro.

03 · Managing digestive effects

Managing nausea, constipation and fatigue

Small habits help most people. Eat smaller meals slowly, stop at the first feeling of fullness, favour plain lower-fat foods while adjusting, stay well hydrated and avoid lying down straight after eating. If nausea or vomiting is persistent or you cannot keep fluids down, contact your clinical team — dehydration needs prompt attention.

Constipation is very common, and may affect more than 1 in 10 people, because food moves through the gut more slowly and many people eat and drink less. Fluids come first, then fibre from vegetables and whole grains, and daily movement. A pharmacist can recommend a suitable laxative if it persists. Contact your clinical team for pain, bloating or more than a few days without a bowel movement.¹

Fatigue is most noticeable in the first weeks and after dose increases. It usually reflects the body adjusting and eating less, so keep meals balanced with protein, stay hydrated and keep gentle activity going. Tell your clinical team if tiredness is severe or not settling.

04 · Hair, face and skin

Hair shedding, face and loose skin

Temporary hair shedding is a recognised side effect, linked to rapid weight loss itself rather than a direct drug effect. For most people it settles as weight stabilises, and protein intake and balanced nutrition matter.¹

"Ozempic face" is not a drug effect. It is what losing a lot of weight quickly can do to facial volume, whatever the method, including diet or surgery. The same applies to loose skin. Adequate protein, resistance exercise and a loss pace guided by your prescriber protect lean mass, and skin commonly adapts gradually once weight stabilises. See weight loss drugs and the face for more.

05 · Serious side effects

Serious side effects: when to seek help

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). Seek immediate medical help for pain in the upper right abdomen, fever or yellowing of the skin or eyes (possible gallbladder problems), or signs of a severe allergic reaction such as swelling of the face or throat and difficulty breathing.¹

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. You can report any suspected side effect through the Yellow Card scheme.¹⁹

Regulators have also looked at low mood and suicidal thoughts. The EMA's safety committee concluded in 2024 that the evidence does not support a causal link, and the MHRA keeps this under review.¹⁷

If you have thoughts of harming yourself or ending your life, get help straight away. Call 999 or go to A&E if you are in immediate danger. In England you can call NHS 111 and choose the mental health option, any time. You can also call Samaritans free on 116 123, day or night. Tell your prescriber about any change in your mood while taking Mounjaro.

06 · Stopping treatment

Stopping because of side effects, and what happens after

Few people stop for this reason. In SURMOUNT-1, 4.3% of people on 5mg, 7.1% on 10mg and 6.2% on 15mg stopped treatment because of side effects, compared with 2.6% on placebo. Stopping for gastrointestinal effects specifically was 1.9% on 5mg, 3.3% on 10mg and 4.3% on 15mg, against 0.5% on placebo.²

Tirzepatide has a half-life of about five days, so after a final dose it takes roughly 4 to 5 weeks to clear the system. Nothing needs doing while it clears, and it cannot be sped up.¹ Speak to your clinical team rather than pushing through severe symptoms. Compare options in our guides to whether prescription weight loss drugs work and Wegovy long-term side effects.