Retatrutide and testosterone are two treatments that operate on distinct but overlapping metabolic pathways, raising important questions about whether they can be safely used together. Retatrutide is an investigational triple incretin receptor agonist — activating GLP-1, GIP, and glucagon receptors — that remains unlicensed in the UK as of mid-2025. Testosterone, by contrast, is a well-established, licensed therapy used for confirmed hypogonadism and gender-affirming care under specialist supervision. This article explores the pharmacology of both agents, potential interactions, who might be considered for concurrent use, and the safety considerations and monitoring requirements that any clinician or patient should understand before combining these treatments.

How Retatrutide and Testosterone Work in the Body

Retatrutide is an investigational triple incretin agonist (GLP-1, GIP, glucagon) that suppresses appetite and may increase energy expenditure, while testosterone is a licensed androgen that influences muscle mass, insulin sensitivity, and fat distribution — both agents affect overlapping metabolic pathways.

Retatrutide is an investigational triple incretin receptor agonist that simultaneously activates glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP), and glucagon receptors. This triple-action mechanism promotes reductions in body weight by suppressing appetite, slowing gastric emptying, and — based on early human data — may increase energy expenditure, though this effect requires further confirmation. As of mid-2025, retatrutide remains in clinical trials and has not received marketing authorisation from the Medicines and Healthcare products Regulatory Agency (MHRA) or the European Medicines Agency (EMA). Phase 2 data published in the New England Journal of Medicine (2023) demonstrated substantial weight loss in adults with obesity, but the medicine is not yet approved for clinical use in the UK.

Testosterone, by contrast, is a well-established androgen hormone used therapeutically in the UK for conditions such as confirmed hypogonadism and, under specialist supervision, as part of gender-affirming hormone therapy (which is generally considered off-label use in the UK and requires specialist oversight). It is available in several licensed UK formulations — principally gels (e.g., Testogel®, Testavan®, Tostran®) and injectable preparations (e.g., Nebido®, Sustanon®) — and works by binding to androgen receptors throughout the body, influencing muscle mass, bone density, red blood cell production, libido, and mood.

Both agents exert metabolic effects. Testosterone influences insulin sensitivity, fat distribution, and lean body mass, while retatrutide targets overlapping metabolic pathways through incretin signalling. Understanding how each drug works individually is essential before considering whether they can be safely used together, particularly given that retatrutide's full interaction and safety profile is still being characterised in ongoing clinical research.

Known and Potential Interactions Between These Medicines

No official interaction data exists between retatrutide and testosterone; the main clinical concern is that retatrutide-induced weight loss may alter SHBG levels, shifting free testosterone ratios and potentially requiring dose adjustments in those on TRT.

Because retatrutide is not yet licensed in the UK, there is currently no official interaction data between retatrutide and testosterone published by regulatory bodies such as the MHRA or EMA. Any discussion of interactions at this stage is therefore based on pharmacological reasoning and extrapolation from related, licensed GLP-1 receptor agonists such as semaglutide (Ozempic®, Wegovy®) and tirzepatide.

One area of theoretical concern relates to altered drug absorption. Retatrutide, like other incretin-based therapies, slows gastric motility. This could potentially affect the absorption of orally administered medications. However, testosterone is most commonly administered via transdermal or injectable routes in the UK, which largely bypasses this concern. Incretin-based agents are also not expected to cause clinically significant cytochrome P450 (CYP)-mediated pharmacokinetic interactions, meaning the metabolism of testosterone is unlikely to be directly affected.

A more clinically relevant consideration involves metabolic and hormonal interplay:

• Significant weight loss induced by retatrutide may alter sex hormone-binding globulin (SHBG) levels, which in turn affects the ratio of free to total testosterone.

• Obesity is associated with lower testosterone levels; therefore, substantial weight reduction may naturally raise endogenous testosterone, potentially requiring dose adjustments in those on testosterone replacement therapy (TRT).

• Both agents can influence insulin sensitivity and cardiovascular risk markers, and their combined effects on lipid profiles and blood pressure are not yet fully characterised.

A pragmatic monitoring approach — checking total testosterone, free testosterone, SHBG, and haematocrit at baseline and at 3–6 months following any significant change in either treatment — is advisable. Dose adjustments should be considered if free testosterone levels or symptoms change materially with weight loss. Patients and clinicians should remain vigilant and monitor hormone levels regularly if both treatments are used concurrently, even in a research context.

Who May Be Prescribed Both Treatments in the UK

Retatrutide cannot currently be prescribed on the NHS as it is unlicensed; access is limited to approved clinical trials, while testosterone is available via NHS or regulated private services for confirmed hypogonadism or specialist-supervised gender-affirming care.

In the UK, testosterone is prescribed through NHS endocrinology, urology, or sexual health services, as well as via regulated private clinics, for individuals with confirmed hypogonadism or as part of gender-affirming care (the latter being off-label and requiring specialist oversight). NICE guidance and British Society for Sexual Medicine (BSSM) guidelines provide clear criteria for initiation and monitoring of testosterone therapy.

Retatrutide, being unlicensed in the UK, cannot currently be prescribed through standard NHS pathways. Access is currently limited to approved clinical trials. Outside of clinical trials, pre-licence access in the UK may theoretically occur via the MHRA's Early Access to Medicines Scheme (EAMS), though no such scheme for retatrutide is publicly available at the time of writing. The MHRA's framework for supplying unlicensed medicinal products ('Specials') and named-patient import routes exist for certain unlicensed medicines, but these mechanisms are generally not applicable to investigational products that are not yet commercially available. Patients interested in trial participation can search for UK studies via the NIHR 'Be Part of Research' portal (bepartofresearch.nihr.ac.uk).

Individuals who might theoretically be candidates for both treatments could include:

• Men with obesity and hypogonadism, where weight loss is a clinical priority alongside hormone replacement

• Transgender men on testosterone therapy who also have obesity-related metabolic conditions

• Individuals enrolled in clinical trials investigating retatrutide's effects on metabolic syndrome, where testosterone use is a pre-existing condition

It is important to note that combining an unlicensed investigational drug with a licensed hormone therapy requires careful oversight. Any such combination should only occur under the supervision of a specialist — such as an endocrinologist or metabolic medicine physician — who can monitor for both efficacy and safety outcomes. Self-prescribing or sourcing retatrutide from unregulated suppliers carries significant health risks and is strongly discouraged.

Risks and Safety Considerations to Discuss With Your Doctor

Key risks include cardiovascular effects from both agents, hormonal fluctuations due to rapid weight loss altering SHBG and free testosterone, gastrointestinal side effects, and the unknown combined safety profile of an unlicensed drug alongside a controlled hormone therapy.

Before considering any combination of retatrutide and testosterone, a thorough risk assessment is essential. Several safety considerations warrant careful discussion with a qualified clinician.

Cardiovascular risk is a primary concern. Testosterone therapy, particularly at supraphysiological doses, has been associated with increased haematocrit, polycythaemia, and potential cardiovascular events. Long-term cardiovascular outcomes data for retatrutide are not yet available; as with other incretin-based therapies, a modest increase in resting heart rate has been observed in early trials. The combined cardiovascular impact of both agents in the same individual therefore remains unknown, and no favourable cardiovascular outcome should be assumed.

Hormonal fluctuations are another key consideration:

• Rapid weight loss can significantly alter SHBG and free testosterone levels, potentially leading to symptoms of androgen excess or deficiency even when the prescribed testosterone dose remains unchanged.

• Regular blood tests — including total and free testosterone, SHBG, haematocrit, liver function, and lipid profile — are advisable. In line with BSSM guidance and testosterone SmPCs (e.g., Nebido®), TRT should be paused or the dose reviewed if haematocrit exceeds 0.54.

• Prostate-specific antigen (PSA) and prostate assessment should be performed as recommended by BSSM guidelines and individual testosterone SmPCs before and during TRT. Testosterone therapy is contraindicated in individuals with known or suspected prostate cancer or male breast cancer.

• Fertility: testosterone therapy suppresses spermatogenesis and can significantly reduce fertility. Individuals who may wish to father children in the future should discuss this with their clinician before starting or continuing TRT.

Gastrointestinal side effects are common with retatrutide and include nausea, vomiting, diarrhoea, and reduced appetite. These effects may affect nutritional status, which in turn can influence hormone metabolism and overall wellbeing. Retatrutide should be used with caution in individuals with known gastroparesis, as slowing of gastric emptying may worsen this condition.

Serious gastrointestinal red flags: if you experience severe, persistent upper abdominal pain — particularly pain radiating to the back, accompanied by vomiting or jaundice — seek urgent medical attention, as these may be signs of pancreatitis or gallbladder disease, which have been associated with incretin-based therapies.

Additionally, individuals should be aware that:

• Retatrutide is not licensed in the UK, meaning its full risk profile is still being established

• Testosterone is a controlled drug in the UK under the Misuse of Drugs Act 1971 when obtained without a valid prescription

• Any combination therapy should be documented, monitored, and reviewed regularly by a specialist

Current MHRA and Clinical Guidance on Combining These Drugs

Neither the MHRA nor NICE has issued specific guidance on combining retatrutide and testosterone; clinicians should seek specialist endocrinology input, consult trial protocols, and follow GMC prescribing guidance when considering this investigational combination.

As of the time of writing, neither the MHRA nor NICE has issued specific guidance on the concurrent use of retatrutide and testosterone. This is primarily because retatrutide has not yet completed the regulatory approval process in the UK or European Union. The MHRA and EMA will only issue formal prescribing guidance — including interaction warnings — once a medicine has been granted a marketing authorisation.

For licensed GLP-1 receptor agonists such as semaglutide (Ozempic®, Wegovy®) and liraglutide (Saxenda®), the MHRA-approved Summaries of Product Characteristics (SmPCs) do not list testosterone as a contraindicated or interacting medicine. Clinicians are nonetheless advised to monitor patients on any hormonal therapy when initiating weight-loss treatments, given the potential for significant physiological changes with substantial weight loss.

NICE technology appraisal TA875 (semaglutide for managing overweight and obesity) provides the current UK framework for licensed incretin-based weight management and focuses on metabolic and cardiovascular outcomes; it does not specifically address concurrent testosterone use. This appraisal is useful context for understanding how similar agents are evaluated in the UK, but it does not apply directly to retatrutide, which remains unlicensed.

Clinicians managing patients on testosterone who wish to explore retatrutide should:

• Consult the relevant clinical trial protocols if the patient is enrolled in a study

• Seek specialist endocrinology input before initiating any off-label or investigational combination

• Document clinical rationale clearly, obtain informed consent, and follow GMC Good Practice in Prescribing and Managing Medicines and Devices guidance when considering unlicensed or investigational treatments

Until formal guidance is available, clinical decision-making should be guided by individual patient risk–benefit assessment, informed consent, and regular monitoring.

When to Seek Medical Advice Before Starting Either Treatment

Anyone on testosterone considering retatrutide — or vice versa — should consult their GP or specialist before proceeding; urgent medical attention is needed for chest pain, severe upper abdominal pain, or symptoms suggesting pancreatitis or cardiovascular events.

Anyone considering retatrutide — whether through a clinical trial or any other route — should consult their GP or a specialist before proceeding, particularly if they are already taking testosterone or any other hormonal therapy. Early medical review helps ensure that treatment decisions are safe, evidence-based, and appropriately monitored.

You should seek medical advice promptly if you:

• Are currently prescribed testosterone and are considering joining a retatrutide clinical trial

• Have noticed symptoms of hormonal imbalance — such as fatigue, mood changes, reduced libido, or unexpected weight changes — while on either treatment

• Have a history of polycythaemia, liver disease, prostate conditions, or cardiovascular disease, as these may affect suitability for either treatment

• Are sourcing retatrutide from unregulated online suppliers, which carries serious risks of counterfeit or contaminated products

Seek urgent medical attention — call 999 or use NHS 111 — if you experience:

• Chest pain, palpitations, or breathlessness

• Severe, persistent upper abdominal pain (particularly if it radiates to the back), vomiting, or jaundice, which may indicate pancreatitis or gallbladder disease

For those already on testosterone therapy through the NHS or a regulated private provider, it is important to inform your prescribing clinician of any new treatments — including investigational drugs — so that monitoring plans can be adjusted accordingly. Your GP can also refer you to an endocrinologist or metabolic medicine specialist if more complex management is required.

Patients are encouraged to report any unexpected side effects from either treatment to the MHRA via the Yellow Card scheme (yellowcard.mhra.gov.uk), which plays a vital role in post-market safety surveillance and helps build the evidence base for emerging therapies such as retatrutide.

Frequently Asked Questions