How many units of retatrutide should be used is a question increasingly asked as interest in this investigational triple receptor agonist grows. Retatrutide is not yet licensed in the UK by the MHRA or EMA, and it is important to clarify that doses are measured in milligrams (mg), not 'units' as used for insulin. Currently available only within Phase 3 clinical trials, retatrutide simultaneously activates GLP-1, GIP, and glucagon receptors, distinguishing it from approved therapies such as semaglutide and tirzepatide. This article explains the dosing framework from Phase 2 trial data, administration guidance, and key safety considerations.

What Is Retatrutide and How Does It Work?

Retatrutide is an investigational triple receptor agonist that activates GLP-1, GIP, and glucagon receptors simultaneously, improving blood glucose control and promoting weight loss. It is not licensed by the MHRA or EMA and remains in Phase 3 clinical investigation.

Retatrutide is an investigational injectable medication under clinical development for the treatment of obesity and type 2 diabetes. It belongs to a novel class of agents known as triple receptor agonists, meaning it simultaneously activates three distinct hormone receptors: the glucagon-like peptide-1 (GLP-1) receptor, the glucose-dependent insulinotropic polypeptide (GIP) receptor, and the glucagon receptor. This triple mechanism of action distinguishes retatrutide from existing approved therapies such as semaglutide (a GLP-1 receptor agonist) or tirzepatide (a dual GIP/GLP-1 receptor agonist).

By activating GLP-1 receptors, retatrutide promotes insulin secretion in a glucose-dependent manner, reduces glucagon release, and slows gastric emptying — all of which contribute to improved blood glucose control and reduced appetite. The additional activation of GIP receptors may enhance insulin sensitivity and further support weight reduction. The glucagon receptor component is hypothesised, based on preclinical and early clinical data, to increase energy expenditure and promote fat breakdown (lipolysis); this is thought to contribute to the substantial weight loss observed in early-phase clinical trials, though the precise human mechanistic contribution remains under investigation.

Retatrutide has not received marketing authorisation from the Medicines and Healthcare products Regulatory Agency (MHRA) or the European Medicines Agency (EMA), and no marketing authorisation application has been approved as of the date of publication of this article. It remains under Phase 3 clinical investigation. Retatrutide is therefore not available as a licensed medicine in the UK, and any use outside of approved clinical trials would be unlicensed and potentially unsafe. Patients should not attempt to source or self-administer this medication outside of a regulated clinical setting.

Understanding Retatrutide Units and Dosage Measurements

Retatrutide is dosed in milligrams (mg), not 'units'; Phase 2 trials tested 1 mg, 4 mg, 8 mg, and 12 mg once-weekly doses. Confusing mg with insulin units could cause dangerous dosing errors.

In clinical trials, retatrutide doses are expressed in milligrams (mg) rather than in 'units' as used for some other injectable medicines such as insulin. This distinction is important: insulin is traditionally measured in international units (IU), whereas peptide-based receptor agonists like retatrutide, semaglutide, and tirzepatide are dosed by weight (milligrams). If you have encountered the term 'units' in relation to retatrutide, this likely reflects informal or colloquial usage, or confusion with insulin dosing terminology.

In the primary Phase 2 clinical trial published in the New England Journal of Medicine (Jastreboff et al., 2023), retatrutide was studied across several dose levels, including:

• 1 mg (low dose)

• 4 mg (mid dose)

• 8 mg (higher dose)

• 12 mg (highest dose)

These doses were administered as once-weekly subcutaneous injections. The trial demonstrated dose-dependent weight loss, with participants receiving 12 mg achieving a mean body weight reduction of approximately 24% over 48 weeks — a result that has attracted considerable scientific and public interest.

Understanding the correct unit of measurement matters for patient safety. Misinterpreting milligrams as 'units' in the insulin sense could lead to significant dosing errors. Until retatrutide receives regulatory approval and a licensed Summary of Product Characteristics (SmPC) is published by the MHRA, there is no officially sanctioned dosing framework for use in routine clinical practice in the UK. All dosing information currently available derives from investigational trial protocols.

Recommended Retatrutide Dose Escalation Schedule

Retatrutide is initiated at a low dose and titrated upward at approximately four-weekly intervals, targeting up to 12 mg once weekly in the highest-dose trial arm. This schedule is investigational only; no licensed UK prescribing framework currently exists.

Based on Phase 2 clinical trial data (Jastreboff et al., NEJM, 2023), retatrutide is administered using a gradual dose escalation schedule. This approach — common to GLP-1 and related receptor agonist therapies — is designed to minimise gastrointestinal side effects such as nausea, vomiting, and diarrhoea, which are among the most frequently reported adverse events with this class of medication.

In the Phase 2 trial, participants began at a lower starting dose and increased incrementally over several weeks before reaching a maintenance dose. The general principle of escalation observed in the trial involved starting at a low dose and titrating upward at approximately four-weekly intervals, with the highest-dose arm targeting 12 mg once weekly. The precise step sizes and timings varied by protocol arm.

This stepwise escalation principle mirrors the approach used for licensed medicines such as semaglutide (Wegovy®, UK SmPC available via the electronic Medicines Compendium) and tirzepatide (Mounjaro®, UK SmPC available via the electronic Medicines Compendium), both of which are approved by the MHRA and recommended within relevant NICE guidance for specific indications.

It is essential to emphasise that these escalation details are derived from investigational trial protocols and do not constitute a licensed prescribing schedule. No UK SmPC exists for retatrutide. The precise escalation regimen for any future approved formulation may differ substantially from trial protocols. Patients should never attempt to self-prescribe or self-escalate doses based on published trial data. Any dose adjustments must be made under the direct supervision of a qualified healthcare professional within an appropriate clinical framework.

How Retatrutide Is Administered: Injection Guidance

Retatrutide is administered as a once-weekly subcutaneous injection into the abdomen, upper thigh, or outer upper arm, with injection sites rotated each week. No commercially available injection device exists in the UK as the medicine is not yet licensed.

In clinical trials, retatrutide has been administered as a subcutaneous injection — meaning it is injected into the fatty tissue just beneath the skin — once weekly. This route of administration is consistent with other approved GLP-1 receptor agonist therapies and is generally well tolerated when performed correctly.

Common injection sites used in trials include:

• The abdomen (at least 5 cm away from the navel)

• The upper thigh

• The upper arm (outer aspect)

Rotating injection sites with each weekly dose is recommended to reduce the risk of localised skin reactions such as bruising or injection site discomfort.

The specific delivery device used varies by trial protocol; participants receive structured training from the trial healthcare team before self-administering. Device type and injection technique should always follow the instructions provided by the trial team, as these may differ between studies. General principles for subcutaneous injection — such as ensuring the injection site is clean and following the device instructions carefully — apply, but patients should not rely on guidance intended for other devices.

Needles and used injection devices must be disposed of safely in an approved sharps container. Follow NHS and local authority waste disposal guidelines; your GP surgery, local pharmacy, or NHS trust can advise on sharps disposal services in your area.

As retatrutide is not yet licensed, no commercially available injection device exists in the UK at this time.

Safety Considerations and Reporting Suspected Side Effects

The most common adverse effects are gastrointestinal, including nausea, vomiting, and diarrhoea, particularly during dose escalation. Suspected adverse reactions should be reported to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk.

Because retatrutide has not received MHRA marketing authorisation, there is currently no official MHRA guidance specifically addressing its dosing, safety profile, or use in clinical practice outside of approved trials. Any retatrutide being offered outside of a regulated clinical trial in the UK is unlicensed, and its quality, safety, and efficacy cannot be guaranteed. The MHRA provides guidance on the use of unlicensed medicines in the UK, which clinicians and patients should consult if relevant.

Based on Phase 2 trial data (Jastreboff et al., NEJM, 2023), the most commonly reported adverse effects of retatrutide include:

• Gastrointestinal effects: Nausea, vomiting, diarrhoea, and constipation (most frequent, particularly during dose escalation)

• Injection site reactions: Redness, bruising, or mild discomfort

• Decreased appetite: Generally considered a therapeutic effect but can contribute to nutritional deficiency if severe

• Increased heart rate: Observed with glucagon receptor agonism; requires monitoring in patients with cardiovascular conditions

The glucagon receptor component of retatrutide raises particular considerations. Unlike pure GLP-1 agonists, glucagon receptor activation can increase hepatic glucose output and may require careful monitoring in patients with diabetes. Patients taking insulin or sulfonylureas alongside any GLP-1–class agent should be aware of an increased risk of hypoglycaemia (low blood sugar); this is a recognised class caution applicable to retatrutide.

Rapid weight loss associated with this class of medication may increase the risk of gallbladder disease, including gallstones. This is a recognised caution for GLP-1 receptor agonist therapies (see semaglutide and tirzepatide UK SmPCs) and is likely relevant to retatrutide.

There is ongoing investigation into potential effects on bone density and thyroid tissue, consistent with theoretical class signals observed with other agents. These risks have not been established for retatrutide in humans and should be regarded as hypothetical at this stage, pending further trial data.

Reporting suspected side effects: Any adverse events occurring within a clinical trial should be reported through the trial's pharmacovigilance framework. In addition, suspected adverse drug reactions to any medicine — including investigational agents — can be reported to the MHRA via the Yellow Card scheme (yellowcard.mhra.gov.uk). Clinicians considering involvement in retatrutide trials should consult the Medicines for Human Use (Clinical Trials) Regulations 2004.

When to Seek Medical Advice About Your Retatrutide Dose

Seek prompt medical advice for persistent vomiting, severe abdominal pain, palpitations, or signs of hypoglycaemia; call 999 or go to A&E for chest pain, anaphylaxis, or sudden severe abdominal pain. Contact NHS 111 if you are unsure whether symptoms require urgent attention.

Given that retatrutide is currently only available within clinical trials, all participants should have direct access to a trial medical team for any concerns relating to dosing or side effects. It is important to understand the circumstances in which prompt medical advice should be sought — both for trial participants and for healthcare professionals monitoring patients.

Contact your trial team or GP promptly if you experience:

• Persistent or severe nausea, vomiting, or diarrhoea that prevents adequate fluid or food intake

• Signs of dehydration, such as dizziness, dark urine, or reduced urination

• Severe or persistent abdominal pain, which may rarely indicate pancreatitis — a known risk with GLP-1 receptor agonist therapies — or gallbladder problems

• Rapid or irregular heartbeat (palpitations)

• Significant injection site reactions, including signs of infection (redness, warmth, swelling, or discharge)

• Symptoms of low blood sugar (hypoglycaemia) if you are also taking insulin or a sulfonylurea, such as shakiness, sweating, or confusion

If you are unsure whether your symptoms require urgent attention, contact NHS 111 (by phone or online at 111.nhs.uk) for advice.

Seek emergency medical attention (call 999 or go to A&E) if you experience:

• Chest pain or difficulty breathing

• Severe allergic reaction (anaphylaxis), including swelling of the face, lips, or throat

• Loss of consciousness or collapse

• Sudden, severe abdominal pain

For patients who have sourced retatrutide outside of a clinical trial — which is strongly discouraged — it is particularly important to inform your GP or a pharmacist, as there is no licensed prescribing information to guide safe use, and the risk of dosing errors or contaminated products is significant.

If you are interested in accessing retatrutide through a legitimate route, speak to your GP or a specialist obesity or diabetes clinic about whether any ongoing clinical trials may be appropriate for you. The NIHR 'Be Part of Research' service (bepartofresearch.nihr.ac.uk) and ClinicalTrials.gov list currently recruiting studies in the UK and can help you identify suitable trials.

Frequently Asked Questions