Retatrutide is an investigational triple incretin receptor agonist targeting GLP-1, GIP, and glucagon receptors, currently in clinical trials for obesity and type 2 diabetes. Knowing what not to take with retatrutide is essential for patient safety, even at this pre-authorisation stage. Because it shares pharmacological features with licensed GLP-1 receptor agonists such as semaglutide and tirzepatide, clinically relevant interactions — including with antidiabetic medicines, anticoagulants, and narrow therapeutic index drugs — can be anticipated. This article outlines key medicines, foods, and supplements to approach with caution, and identifies patient groups who should seek specialist advice before use.

Medicines That May Interact With Retatrutide

The most significant interactions involve insulin and sulfonylureas, which may cause hypoglycaemia when combined with retatrutide; caution is also warranted with anticoagulants, oral contraceptives, and other GLP-1 receptor agonists.

Retatrutide is an investigational triple incretin receptor agonist — acting on GLP-1, GIP, and glucagon receptors — currently undergoing clinical trials for the treatment of obesity and type 2 diabetes. It has not yet received marketing authorisation from the Medicines and Healthcare products Regulatory Agency (MHRA) or the European Medicines Agency (EMA). Definitive interaction guidance will only be available once a UK Summary of Product Characteristics (SmPC) is published; the information below is extrapolated from authorised GLP-1 and GLP-1/GIP receptor agonists (such as semaglutide and tirzepatide) and should be interpreted accordingly.

The most clinically significant interaction concern involves antidiabetic medicines. Because retatrutide lowers blood glucose through multiple hormonal pathways, combining it with insulin or sulfonylureas (such as gliclazide or glibenclamide) may substantially increase the risk of hypoglycaemia. Dose adjustments of these agents are likely to be necessary under medical supervision.

Other medicines warranting caution include:

• Oral contraceptives — the tirzepatide SmPC (Mounjaro, MHRA/emc) notes a reduction in oral contraceptive exposure during initiation and dose escalation, and advises considering additional contraceptive precautions during this period. Whether retatrutide carries the same risk is not yet established; patients using oral hormonal contraceptives should discuss this with their prescriber pending a retatrutide SmPC.

• Warfarin and other anticoagulants — in line with GLP-1 RA SmPCs (e.g., semaglutide), more frequent INR monitoring is advisable when starting retatrutide or changing the dose, as any alteration in gastric motility could affect anticoagulant control.

• Other GLP-1 receptor agonists — combining retatrutide with another GLP-1 RA is not recommended, as this is not supported by clinical evidence and may increase the risk of adverse effects.

Patients and prescribers should consult the most current SmPC once available, alongside relevant NICE guidance, before initiating retatrutide alongside any existing medication regimen.

Foods, Supplements and Substances to Avoid

Alcohol may worsen gastrointestinal side effects and compound hypoglycaemia risk when retatrutide is used with insulin or sulfonylureas; herbal supplements with hypoglycaemic properties such as berberine or fenugreek should also be used with caution.

Whilst retatrutide is not yet licensed in the UK, its pharmacological profile — particularly its effect on slowing gastric emptying and reducing appetite — has implications for how certain foods, supplements, and substances may interact with the medicine.

Alcohol warrants consideration. The primary hypoglycaemia risk from alcohol is most relevant when retatrutide is used alongside insulin or sulfonylureas, as alcohol can independently lower blood glucose and may compound the glucose-lowering effects of these agents. Even without such combinations, alcohol can irritate the gastrointestinal tract, and since nausea, vomiting, and gastrointestinal discomfort are among the most commonly reported side effects of GLP-1-based therapies, alcohol consumption may worsen these symptoms. Moderation is advisable, and patients should follow NHS guidance on safe alcohol limits.

With regard to dietary supplements, patients should be aware of the following:

• Herbal supplements with hypoglycaemic properties — such as berberine, bitter melon, or fenugreek — may enhance blood glucose-lowering effects and increase hypoglycaemia risk, particularly when used alongside insulin or sulfonylureas.

• High-fat or high-sugar meals — whilst not contraindicated, these may worsen gastrointestinal side effects, particularly during the dose escalation phase. A balanced, nutrient-rich diet is recommended in line with NHS dietary guidance for weight management and diabetes.

Patients are advised to discuss any supplements they are taking with their GP or pharmacist before starting retatrutide. There is no formally established link between specific foods and serious adverse outcomes with retatrutide at this stage, but caution is prudent given the drug's mechanism of action.

How Retatrutide Affects Other Medications in Your Body

Retatrutide may delay gastric emptying, potentially altering the absorption of co-administered oral medicines; narrow therapeutic index drugs such as digoxin, ciclosporin, and antiepileptics warrant closer monitoring.

One of the most pharmacologically relevant features of retatrutide — shared with other GLP-1 receptor agonists such as semaglutide and liraglutide — is its potential to delay gastric emptying. This means that food and orally administered medicines may move more slowly through the stomach and into the small intestine, where most drug absorption occurs. This could theoretically alter the peak plasma concentration (Cmax) and time to peak concentration (Tmax) of co-administered oral drugs. It is worth noting, however, that gastric emptying effects with incretin therapies may attenuate with continued dosing, and the clinical relevance of any absorption change varies considerably between medicines.

Medicines with a narrow therapeutic index — where small changes in blood concentration can have significant clinical consequences — merit particular attention. Based on class data from authorised GLP-1 RA SmPCs, prescribers should consider therapeutic drug monitoring or clinical assessment when initiating retatrutide in patients taking medicines such as:

• Digoxin — used for heart failure and atrial fibrillation.

• Ciclosporin — an immunosuppressant where consistent drug levels are critical.

• Antiepileptic drugs such as phenytoin or carbamazepine — where subtherapeutic levels could precipitate seizures.

These are precautionary considerations extrapolated from class data; GLP-1 RA SmPCs (e.g., semaglutide) generally report no clinically relevant effect on overall drug exposure for most oral medicines, but individual monitoring remains prudent for narrow therapeutic index agents.

Claims that retatrutide's glucagon receptor activity may interact with statins or fibrates are not currently supported by published clinical data and are not included here.

Patients should never adjust or discontinue any prescribed medication without first consulting their GP or specialist. Pharmacists can provide valuable guidance on monitoring and the management of any potential interactions.

Who Should Exercise Extra Caution or Avoid Retatrutide

Patients with a history of medullary thyroid carcinoma, MEN2, pancreatitis, severe gastroparesis, or pregnancy should exercise particular caution or avoid retatrutide until further safety data are available.

Certain patient groups may face a higher risk of adverse effects or complications with retatrutide and should approach its use with particular caution, or may be advised to avoid it altogether pending further evidence.

Patients with a personal or family history of medullary thyroid carcinoma (MTC) or Multiple Endocrine Neoplasia type 2 (MEN2) should seek specialist advice before using any incretin-based therapy. Preclinical studies in rodents have shown an association between GLP-1 receptor activation and thyroid C-cell changes, although a direct causal link in humans has not been established, and UK/EU SmPCs for authorised GLP-1 RAs do not list MTC or MEN2 as a formal contraindication. Patients and clinicians should remain alert to symptoms such as a new neck lump, hoarseness, or difficulty swallowing, and seek prompt medical assessment if these occur.

Other groups requiring careful evaluation include:

• Patients with a history of pancreatitis — a causal relationship between incretin-based therapies and pancreatitis has not been definitively established, but UK SmPCs advise caution in those with a prior history. Treatment should be stopped and urgent medical attention sought if symptoms of pancreatitis occur (see the section on reporting side effects below).

• Individuals with severe gastroparesis — delayed gastric emptying could worsen symptoms in this group, and retatrutide is likely to be unsuitable.

• Pregnant or breastfeeding women — there is currently insufficient safety data to support use during pregnancy or lactation.

• Patients with severe renal or hepatic impairment — whether dose adjustments or avoidance are necessary will depend on the pharmacokinetic data presented in the eventual retatrutide SmPC.

• Children and adolescents — retatrutide has not been studied in paediatric populations and should not be used outside of approved clinical trial settings.

A thorough medical history and clinical assessment by a qualified prescriber are essential before initiating treatment.

Guidance From Your GP or Prescriber Before Starting Treatment

A comprehensive medicines reconciliation review with your GP or specialist is essential before starting retatrutide, covering all current medicines, supplements, and relevant medical history.

Before starting retatrutide — or any new medicine — it is essential to have a comprehensive consultation with your GP, specialist, or prescriber. This is particularly important given that retatrutide is still in clinical development and is not yet available as a licensed treatment through the NHS or private prescribing channels in the UK. Patients should be cautious of unregulated sources offering retatrutide outside of approved clinical trials.

During your consultation, your prescriber should conduct a thorough medicines reconciliation review, which involves:

• Listing all current prescription medicines, including inhalers, patches, and injections.

• Reviewing over-the-counter medicines and any herbal or nutritional supplements.

• Assessing your medical history for conditions that may increase the risk of interactions or adverse effects.

• Discussing lifestyle factors, including alcohol intake, diet, and physical activity levels.

Your prescriber may arrange baseline investigations tailored to your individual circumstances and the indication for treatment. In line with NICE guidance (e.g., NG28 for type 2 diabetes; TA875 for weight management), these may include blood glucose monitoring, HbA1c (where relevant), and renal and liver function tests where clinically indicated. Routine thyroid function testing is not a standard requirement before starting GLP-1 or GIP-based therapies in UK practice, but may be arranged if there is a specific clinical reason.

If you use oral hormonal contraceptives, discuss this with your prescriber, as data from the tirzepatide SmPC suggest that additional contraceptive precautions may be advisable during initiation and dose escalation of GLP-1/GIP-based therapies, pending specific retatrutide data.

If you are currently taking medicines with a narrow therapeutic index, your prescriber may recommend closer monitoring during the initiation and dose escalation phases. It is important to attend all follow-up appointments and to report any new or worsening symptoms promptly. Never start, stop, or adjust any medication without professional advice, and always inform any other healthcare professional — including dentists and pharmacists — that you are taking retatrutide.

Reporting Side Effects and Interactions in the UK

Suspected adverse reactions to retatrutide should be reported via the MHRA Yellow Card scheme; trial participants must also report directly to the trial sponsor and principal investigator.

In the UK, the reporting of suspected side effects and drug interactions is a vital component of ongoing drug safety monitoring. The MHRA operates the Yellow Card scheme, which allows patients, carers, and healthcare professionals to report suspected adverse drug reactions (ADRs) and interactions. Reports can be submitted online at yellowcard.mhra.gov.uk, via the Yellow Card app, or through a paper form available from pharmacies.

For retatrutide specifically, as it remains an investigational medicine, any adverse events experienced within a clinical trial setting should be reported directly to the trial sponsor and principal investigator in accordance with the trial protocol. Regulatory oversight of clinical trials in the UK is managed by the MHRA in conjunction with the Health Research Authority (HRA).

Patients and healthcare professionals should seek urgent medical attention if any of the following occur:

• Signs of severe hypoglycaemia — confusion, loss of consciousness, or seizures.

• Symptoms suggestive of pancreatitis — severe, persistent abdominal pain radiating to the back, with or without vomiting.

• Severe or prolonged vomiting with signs of dehydration — such as dizziness, reduced urine output, or extreme thirst — which may indicate a risk of acute kidney injury.

• Allergic reactions — including rash, swelling of the face or throat, or difficulty breathing.

• Unexplained neck lumps, hoarseness, or difficulty swallowing, which should be assessed promptly.

Healthcare professionals are encouraged to report all suspected interactions and ADRs via the Yellow Card scheme, even if causality is uncertain. Every report contributes to building the evidence base for drug safety in the UK population as new medicines like retatrutide move closer to clinical use.

Frequently Asked Questions