Mounjaro Side Effects: How Common They Are, How Long They Last and When to Get Help

In the weight management trials, more than half of people taking Mounjaro had stomach or gut side effects. Most were mild or moderate and became less common over time. This guide sets out how common each side effect is, using the frequencies in the UK product information for people taking it for weight management, which ones need urgent attention and how to report them.

Mounjaro Side Effects at a Glance

Very common: nausea, diarrhoea, vomiting, constipation and stomach pain. Pancreatitis is uncommon.

The Summary of Product Characteristics (SmPC) for Mounjaro groups side effects by how often they happened in clinical trials. For people taking it for weight management:

• Very common (more than 1 in 10 people): feeling sick (nausea), diarrhoea, vomiting, constipation and stomach (abdominal) pain.

• Common (up to 1 in 10 people): indigestion, bloating, burping, wind, acid reflux, gallstones, tiredness, dizziness, low blood pressure, hair loss, injection site reactions, allergic (hypersensitivity) reactions and a rise in a blood marker called calcitonin.

• Uncommon (up to 1 in 100 people): acute pancreatitis, slower stomach emptying, inflamed gallbladder (cholecystitis), changes in taste, unusual skin sensations, injection site pain and a faster heart rate.

• Rare (up to 1 in 1,000 people): severe allergic reactions (anaphylaxis) and swelling under the skin (angioedema), reported after the medicine came onto the market.

Low blood sugar (hypoglycaemia) is mainly a risk for people with type 2 diabetes who also take insulin or a sulphonylurea.

Stomach and Gut Side Effects

About 56% had stomach side effects (30% on placebo); nausea 24.6-29.0%, diarrhoea 18.7-22.5%; mostly mild or moderate.

These are the side effects people notice most. In pooled data from two weight management trials (SURMOUNT-1 and SURMOUNT-2), reported in section 4.8 of the SmPC, stomach and gut side effects affected about 56% of people on tirzepatide at each dose (5 mg, 10 mg and 15 mg), compared with about 30% on placebo:

• Nausea: 24.6% to 29.0% across the doses, against 8.5% on placebo.

• Diarrhoea: 18.7% to 22.5%, against 7.8% on placebo.

Most were mild (61%) or moderate (34%). The SmPC notes that nausea, vomiting and diarrhoea were more common while the dose was being increased and became less common over time.

Vomiting and diarrhoea can lead to dehydration, which can affect the kidneys. The SmPC advises taking care to avoid losing too much fluid, especially in older people, so keep drinking fluids and speak to your prescriber or a pharmacist if you cannot keep fluids down.

Other Common Side Effects

Burping, indigestion, tiredness, dizziness, hair loss (4.9%), injection site reactions (8.0%) and gallstones.

The figures below are from the pooled weight management trial data in section 4.8 of the SmPC.

• Burping, indigestion, bloating, wind and reflux are all listed as common.

• Tiredness (including fatigue, weakness and feeling generally unwell) is common.

• Dizziness and low blood pressure are common in the weight management trials; 94% of the low blood pressure events were mild or moderate.

• Hair loss was reported by 4.9% of people on tirzepatide and 1.0% on placebo in the weight management trials. It was mainly mild, and most people recovered while still taking it.

• Injection site reactions, mostly redness and itching, were reported by 8.0% of people on tirzepatide and 1.8% on placebo in the weight management trials, and all were mild or moderate.

• Gallstones are common in the weight management trials. The SmPC notes that gallbladder problems were linked to the amount of weight lost.

Serious Side Effects: When to Get Help

Persistent, severe stomach pain or signs of a severe allergic reaction need immediate help.

Some side effects are rare but need urgent attention:

• Acute pancreatitis (inflammation of the pancreas) is uncommon. The SmPC says to seek immediate medical attention for persistent, severe stomach pain. If pancreatitis is confirmed, Mounjaro should not be restarted.

• Severe allergic reactions, such as swelling of the face, lips, tongue or throat or difficulty breathing, need emergency help. Anaphylaxis and angioedema are rare.

• Gallbladder inflammation (cholecystitis) is uncommon in the weight management trials; tell your prescriber about any new or severe stomach pain.

• Low blood sugar is a risk mainly with insulin or a sulphonylurea; your prescriber may lower those doses.

• Dehydration from vomiting or diarrhoea can affect the kidneys.

• Diabetic eye disease: the SmPC notes that a rapid improvement in blood sugar control has been linked to a temporary worsening of diabetic retinopathy, so people with pre-existing diabetic retinopathy should be monitored closely.

If you think you are having a serious reaction, call 999 or go to A&E. For urgent advice that is not an emergency, call NHS 111.

Side Effects If You Do Not Have Diabetes

The SmPC's weight management frequencies come from trials that mostly included people without diabetes.

The frequencies in this guide are the SmPC's figures from the weight management trials, which mostly included people without diabetes. Vomiting, constipation, stomach pain and gallstones fall into a higher frequency band in these trials than in the type 2 diabetes trials.

Low blood sugar is mainly a concern for people who also take insulin or a sulphonylurea, which most people taking Mounjaro for weight management do not.

How Long Do Mounjaro Side Effects Last?

Stomach side effects are worst while the dose increases and ease over time.

For most people, stomach side effects are worst in the weeks after starting and after each dose increase, and ease over time. That is why Mounjaro starts at 2.5 mg and goes up in small steps, at least 4 weeks apart.

If side effects do not settle or are hard to manage, speak to your prescriber before your next dose increase. They may suggest staying on your current dose for longer.

Pregnancy, Breastfeeding and Contraception

Not in pregnancy; stop 1 month before a planned pregnancy; extra contraception for 4 weeks after starting and each increase.

• Pregnancy: Mounjaro should not be used during pregnancy. If you want to become pregnant, the SmPC advises stopping it at least 1 month before, because it stays in the body for a long time.

• Contraceptive pills: Mounjaro may make the pill less effective. The SmPC advises switching to a non-oral method, or adding a barrier method such as condoms, for 4 weeks after starting and for 4 weeks after each dose increase.

• Breastfeeding: the SmPC says Mounjaro could be considered, as only very low amounts were found in breast milk. Discuss it with your prescriber.

Before an Operation or Procedure

Tell the team you take Mounjaro, because it slows stomach emptying.

Mounjaro slows down how quickly the stomach empties. The SmPC notes reports of food or fluid being breathed into the lungs (pulmonary aspiration) in people taking this type of medicine who had a general anaesthetic or deep sedation. If you are due to have an operation or procedure, tell the team that you take Mounjaro.

Thyroid and Cancer Questions

Thyroid C-cell tumours were seen in rats; the SmPC says relevance to humans is unknown.

In a 2-year study in rats, tirzepatide caused an increase in thyroid C-cell tumours. The SmPC states that the relevance of this to humans is unknown, and the same effect was not seen in a six-month study in mice. A rise in blood calcitonin, a marker linked to these cells, is listed as a common finding in the weight management trials.

Tell your prescriber about your medical history, including any thyroid conditions, before starting.

Reporting Side Effects

Tell your prescriber and report suspected side effects to the MHRA Yellow Card scheme.

Tell your prescriber or pharmacist about any side effect that bothers you or does not go away. You can also report suspected side effects to the MHRA Yellow Card scheme, online or through the Yellow Card app. Reports help keep medicines safe for everyone.

Frequently Asked Questions