Retatrutide vs Tirzepatide Side Effects Comparison (October 2026)

01 · The verdict

Which has worse side effects, tirzepatide or retatrutide?

Nobody can honestly rank them for a UK patient today. Tirzepatide has a licensed, regulator-reviewed side-effect profile with published rates. Retatrutide has no UK licence, so there is no approved UK product information to quote and no regulated supply.

Tirzepatide's side effects are mostly digestive, they cluster around starting treatment and dose increases, and for most people they ease within days to a few weeks.¹ Mounjaro is a licensed UK treatment for weight management in adults, supplied after a free online consultation reviewed by a GPhC-registered prescriber. Retatrutide, sold online as a 'research' product, is not.

Putting two separate trials' numbers side by side is an indirect comparison, and trial populations, designs and doses differ.

02 · Who each option suits

Should I switch from tirzepatide to retatrutide?

No. Switching from a licensed, prescriber-supervised medicine to an unlicensed product with no UK safety information would swap known, manageable side effects for unknown ones. If your current dose is hard to tolerate, the better first step is to speak to your clinical team: prescribers can hold the current dose longer before stepping up.¹

If you are weighing licensed options, the real choice is between Mounjaro and Wegovy. See our guides on tirzepatide vs semaglutide side effects and retatrutide vs Mounjaro side effects for the wider picture.

Tirzepatide suits adults who want a once-weekly licensed treatment backed by large trials and who are comfortable with a gradual dose ladder. You can see current strengths and prices on the Mounjaro pen page.

03 · Decision factors

What to weigh up before choosing

Four factors matter more than any headline comparison. First, licensing: a licensed medicine has its side effects reviewed by regulators and listed in product information you can read. Second, supply: genuine pens from a registered pharmacy mean you know what you are injecting. Third, oversight: ongoing monthly reviews, dose-titration and side-effect support from a prescribing team. Fourth, your own health profile: conditions, other medicines and tolerance decide what suits you.

Free next-day tracked delivery in temperature-controlled packaging, same-day dispatch when ordered by 3pm, and official LillyDirect supply are included. The consultation is free, and a free online consultation is how you find out whether treatment is suitable. If it is not, your order is refunded automatically.

04 · Tirzepatide side-effect rates

Tirzepatide side-effect rates in the trials

Tirzepatide's side effects are mostly digestive, and the published rates are specific. Like all medicines, Mounjaro can cause side effects. The most common, affecting more than 1 in 10 people, are nausea, diarrhoea, vomiting, constipation and abdominal discomfort.¹ Common effects, affecting up to 1 in 10 people, include indigestion, reduced appetite, dizziness, reflux, burping, hair shedding and injection-site reactions.¹³

In the trials supporting Mounjaro's weight-management licence, nausea was reported by 24.6% of people on 5mg, 29.0% on 10mg and 28.0% on 15mg, compared with 8.5% on placebo. Diarrhoea was reported by 18.7% of people on 5mg, 20.8% on 10mg and 22.5% on 15mg, compared with 7.8% on placebo.¹ Notice that nausea did not climb steadily with dose: the 10mg figure was slightly higher than the 15mg one. Dose alone does not predict how you will feel.

Nausea, vomiting and diarrhoea were more common while the dose was being increased and became less frequent over time.¹ That pattern is why treatment uses a gradual ladder rather than starting high. It also means the first weeks, and each step up, are when you are most likely to notice symptoms.

Tolerability can also be measured by how many people stop. Because of gastrointestinal side effects, 1.9% of people on 5mg, 3.3% on 10mg and 4.3% on 15mg stopped treatment permanently, compared with 0.5% on placebo.¹ In SURMOUNT-1, 4.3% on 5mg, 7.1% on 10mg and 6.2% on 15mg stopped treatment because of side effects of any kind, compared with 2.6% on placebo.² So the large majority of people in these trials stayed on treatment through the dose changes.

When a page quotes precise retatrutide percentages or 'mild' side-effect claims, ask where the figures come from and whether they sit in UK product information. For a closer look at how tirzepatide compares with the other licensed injection, see tirzepatide vs semaglutide gastrointestinal side effects.

One caution on reading these numbers: trial averages describe groups, not individuals. Someone on 15mg may have no nausea at all, and someone on 5mg may find it hard. Your prescriber can adjust the pace of your titration to fit.

05 · What we can say about retatrutide

What can honestly be said about retatrutide

A licensed medicine comes with a Summary of Product Characteristics and a patient leaflet that list side effects, rates, interactions and warnings, reviewed by regulators. Retatrutide has no equivalent UK document, so there is no approved UK side-effect list to compare with Mounjaro's.

It is also sold online as a 'research' peptide. Mounjaro must not be combined with unlicensed 'research' peptides such as retatrutide, because these combinations have no safety data and no regulated supply chain.⁷ Products of this kind are outside the checks that apply to a registered pharmacy: you cannot be sure of the strength, the purity or what is actually in the vial.

If you have read that one drug has 'fewer side effects', treat it as an unverified claim, not a comparison.

Is retatrutide safe? Products sold online as 'research' retatrutide sit outside regulated supply, with no prescriber monitoring.

A sensible way to read any retatrutide discussion: ask four questions. Is it licensed where I live? Is there product information I can read? Who is supplying it and are they registered? Will a clinician review me if something goes wrong? For tirzepatide from us, the answers are yes, yes, a GPhC-registered pharmacy (registration 9012878), and yes. For a 'research' peptide, the answers are no across the board.

If you are curious about the wider field, our guide on retatrutide vs semaglutide and tirzepatide covers how the options are positioned, and we update our guidance when UK licensing status changes.

06 · Managing nausea and digestion

Managing nausea and digestive effects

Most digestive effects on tirzepatide can be managed with a few practical habits. Practical steps that help most people are to 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.¹

The timing pattern is the key to expectation-setting. Most side effects appear in the first days after starting or increasing a dose, peak early, and settle as your body adapts. This is one reason Mounjaro uses a gradual dose ladder.¹ If you know a step up is coming, it helps to plan lighter meals around the injection day and keep water to hand.

If a new dose is hard to tolerate, prescribers can hold the current dose longer before stepping up. Speak to your clinical team rather than pushing through severe symptoms.¹ This flexibility is an advantage of a supervised pathway: the pace is set around you, not around a schedule. Our ongoing monthly reviews and dose-titration follow-up exist precisely for this.

Constipation deserves its own mention. Constipation is very common on Mounjaro for weight management and may affect more than 1 in 10 people, because food moves through the gut more slowly and many people eat and drink less. What helps is fluid first (regular water through the day), then fibre from vegetables and whole grains, and daily movement. If it persists, a pharmacist can recommend a suitable laxative. Contact your clinical team for pain, bloating or more than a few days without a bowel movement.¹

Dehydration is the one digestive risk to take seriously. If nausea or vomiting is persistent or you cannot keep fluids down, contact your clinical team — dehydration needs prompt attention.¹ Do not wait it out.

Remember the pattern in the numbers: the trial rates of nausea and diarrhoea fell over time as people adjusted.¹ So an unpleasant first fortnight at a new dose is common, usually temporary, and a reason to talk to your prescriber, not a reason to self-adjust.

To see how this plays out against the other licensed injection, read our tirzepatide vs semaglutide dosage comparison.

07 · Serious side effects

Serious side effects and when to seek help

Serious side effects on tirzepatide are uncommon, but they need urgent attention, and knowing the signs matters more than any percentage.

Seek immediate medical help for severe, persistent stomach pain that may spread to your back, with or without vomiting, which could be pancreatitis.¹³ Get medical help straight away for pain in the upper right abdomen, fever or yellowing of the skin or eyes, which could signal gallbladder problems.¹³ Get medical help straight away for signs of a severe allergic reaction, such as swelling of the face or throat and difficulty breathing.¹³

If you take insulin or a sulfonylurea alongside Mounjaro, watch for low blood sugar: shakiness, sweating and confusion. Your prescriber may need to adjust those medicines.¹ This is a dose-adjustment conversation, not a ban, so tell your prescriber about everything you take when you complete your consultation.

This is where licensing and supervision earn their keep. With a licensed medicine, the warning signs are written in the patient leaflet, your prescriber knows your history, and you have a route to ask questions. With an unlicensed product, you may not know what to watch for, because there is no approved product information to tell you. And if you are already injecting something unregulated, tell your prescriber honestly, because that affects their safety assessment.

You can also report any suspected side effect, for any medicine, through the MHRA Yellow Card scheme.¹⁹ Reports help regulators spot patterns. If you notice a symptom that is not listed in the patient leaflet, it is still worth discussing with your prescriber and reporting.

Practical rule of thumb: mild, passing digestive symptoms at a dose change are expected and manageable; for new or lasting symptoms, tell your prescriber. If the pain is severe, or you have trouble breathing, get medical help straight away and do not wait for a reply from us.

08 · Hair, skin, fatigue and injection sites

Hair loss, fatigue, injection sites and 'Ozempic face'

Beyond the stomach, the effects people ask about most are hair shedding, tiredness and injection-site reactions, and each has a clear explanation. Hair loss was reported by 4.9% of people taking tirzepatide in the weight-management trials, compared with 1.0% on placebo. Injection-site reactions were reported by 8.0%, compared with 1.8% on placebo.¹

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. Protein intake and overall nutrition matter, which is another reason the reduced-calorie diet should stay balanced.¹

Fatigue is a recognised common side effect, most noticeable in the first weeks and after dose increases. It usually reflects a mix of the body adjusting and simply eating less, so keep meals balanced (protein especially), stay hydrated, and keep gentle activity going. For most people energy returns to normal within a few weeks. Tell your clinical team if tiredness is severe or not settling.¹³

Mild redness, itching, a small bruise or a tender lump at the injection site is common and normally settles within a few days. Rotating sites each week and letting the pen warm briefly out of the fridge reduces stinging. Contact your clinical team if a reaction is spreading, hot, very painful, or lasts more than a week.¹³

'Ozempic face' is not a drug effect. It is what losing a significant amount of weight quickly can do to facial volume, on any method including diet or surgery, and the same applies to loose skin. The counter-measures are the ones that protect lean mass: adequate protein, resistance exercise, and a loss pace guided by your prescriber rather than maximum speed. Skin commonly adapts gradually after weight stabilises.¹

With Mounjaro you can read the rates, and your prescriber can help you plan around them. Needles, a sharps bin and injection guidance are included with every order.

09 · Mood and mental health

Mood, mental health and reporting concerns

Regulators have looked specifically at mood with GLP-1 medicines, and the review did not find a causal link. The EMA's safety committee reviewed the evidence and concluded in 2024 that it does not support a causal link between GLP-1 medicines and low mood or suicidal thoughts, and the MHRA keeps this under continuous 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.

That said, any big change in weight, appetite and routine can affect how you feel. Tell your prescriber about any history of depression before starting, and seek help promptly if your mood drops or you have distressing thoughts during treatment.¹⁷

For tirzepatide, a regulator-level review exists, and ongoing monitoring is in place.

If you do have a symptom you are unsure about, you have two routes. Contact your prescribing team, who can review your dose and history, and report it through the MHRA Yellow Card scheme so it contributes to safety monitoring.¹⁹ Mention it, and let a clinician decide whether it is related.

Support matters here too. Our support team is available Monday to Friday, 9am to 5pm by email at support@everydaymeds.co.uk, through the account portal chat, or by phone on 0333 207 0413. Ongoing monthly reviews include side-effect support, and repeat orders re-check suitability, with no auto-shipping, so someone looks at how you are doing before each new pen.

The free GLPTrackr companion app (iOS and Android) can help you log doses and symptoms, which makes conversations with your prescriber more precise: 'nausea two days after the step up' is far more useful than 'I felt unwell'.

10 · Efficacy: what trials show

Efficacy: what the licensed trials show

Side effects only make sense next to benefit, and tirzepatide's benefit is documented in large trials. In SURMOUNT-1, which enrolled 2,539 adults, people taking tirzepatide alongside diet and exercise lost on average 15% of their body weight on 5mg, 19.5% on 10mg and 20.9% on 15mg over 72 weeks, compared with 3.1% with lifestyle changes alone.² These are trial averages, and individual results vary. Treatment works alongside a reduced-calorie diet and increased physical activity, not instead of them.

To put that in concrete terms, for someone starting at 100 kg, 20.9% is about 21 kg. That is straight arithmetic from the trial average, not a prediction of your result, and trial averages do not predict your own result.

At 72 weeks, 85% of participants on 5mg, 89% on 10mg and 91% on 15mg had lost at least 5% of their body weight. Half of participants on 10mg and 57% on 15mg lost at least 20% of their body weight.²

SURMOUNT-5 was the first large head-to-head trial of tirzepatide and semaglutide for weight loss. In SURMOUNT-5, adults on tirzepatide lost on average 20.2% of body weight over 72 weeks versus 13.7% on semaglutide 2.4mg (Wegovy).¹⁵ In the separate STEP 1 trial, adults on semaglutide 2.4mg lost on average 14.9% of body weight over 68 weeks, compared with 2.4% on placebo.⁹ Both are effective, licensed treatments, and which suits you depends on your health profile, tolerance and prescriber advice, not the headline number alone.

If you want the full efficacy picture across the options, our pages on retatrutide, tirzepatide and semaglutide fat loss and weight loss efficacy and side effects go further, with the same strict approach to what each trial can and cannot show.

11 · Switching, stacking and washout

Switching medicines and why stacking is unsafe

Can you take reta and Mounjaro together? No. Mounjaro must not be combined with other GLP-1 medicines, unlicensed 'research' peptides (such as retatrutide, which has no UK licence), or compounded copies. These combinations have no safety data and no regulated supply chain. If something promises faster results outside a licensed pathway, that is the warning sign.⁷

Stacking also makes it impossible to tell which product is causing a symptom, which is exactly what your prescriber needs to know.

Switching between licensed medicines is a supervised process. There is no licensed direct conversion between semaglutide and tirzepatide. Typically the previous medicine is stopped, a washout interval observed, and Mounjaro started at a dose your prescriber judges appropriate, often the 2.5mg starting dose, sometimes adjusted based on your history. Never run two GLP-1 medicines together.¹

Timing is worth understanding. 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. There is no way to speed this up, and nothing needs doing while it clears.¹ If you have been using an unlicensed product and want to move to a licensed pathway, be open about it in your consultation so a prescriber can advise safely.

Ozempic is semaglutide licensed for type 2 diabetes. It is not a UK weight-loss medicine, and prescribing it privately for weight loss is off-label. The licensed weight-management options are Mounjaro (tirzepatide) and Wegovy (semaglutide 2.4mg), so 'Mounjaro vs Ozempic' for weight loss is really Mounjaro vs Wegovy.¹

Wegovy's pens run 0.25mg, 0.5mg, 1mg, 1.7mg and 2.4mg once weekly, stepping up after at least 4 weeks to reach 2.4mg from week 17. You can view the Wegovy pen if that is the route you are considering, and see all weight loss treatments we offer.

12 · NHS, private cost and ordering

NHS access, private cost and how ordering works

Mounjaro is available on the NHS, but access is being phased in over several years. NICE recommends it for adults with a BMI of at least 35 plus at least one weight-related condition, and the NHS primary-care rollout began in June 2025 with the highest-need groups first, initially BMI 40+ with four or more qualifying conditions.⁴ Full rollout is expected to take years, which is why many people who meet the licensed criteria choose regulated private treatment rather than waiting.⁶ Retatrutide has no NHS or UK pathway at all.

The consultation is free and the price shown is the price paid, with no hidden fees. You can pay by Visa or Mastercard, Klarna or pay-monthly options, with no subscription.

How ordering works, in three steps:

• Free online consultation: a short health questionnaire, no charge and no obligation, via our consultation page.
• Prescriber review: reviewed by a GPhC-registered prescriber, not a scoring script. Payment is taken when you place the order; if the prescriber decides the treatment is not suitable, the order is refunded automatically in full.
• Dispatch and delivery: dispensed by our registered pharmacy, with same-day dispatch when ordered by 3pm and free next-day tracked DPD delivery in discreet, temperature-controlled packaging. Saturday delivery is £14.99.

Everything is genuine Mounjaro KwikPens from official LillyDirect supply, which is the opposite of the unverified vials sold as 'research' products. Our pharmacy can be checked on the GPhC register (9012878), and repeat orders re-check suitability each time.