GLP-1 Medication Interactions

Mounjaro (tirzepatide), Wegovy (semaglutide) and other GLP-1 medicines have relatively few direct chemical interactions with other drugs, but they do interact in important practical ways. They slow stomach emptying, which can change how some tablets are absorbed, most notably the contraceptive pill with tirzepatide. They lower blood glucose, which matters if you take insulin or a sulfonylurea. And their side effects can affect how other medicines behave. Before starting, give your prescriber a complete list of everything you take, and check with a pharmacist before adding anything new.

How GLP-1 medicines interact with other drugs

It helps to understand the three main ways interactions arise, because they explain why some medicines matter and others do not.

1. Slower stomach emptying

GLP-1 receptor agonists slow the rate at which the stomach passes its contents into the small intestine. Most tablets are absorbed in the small intestine, so a slower stomach can delay absorption and change the peak level a medicine reaches in the blood. For most medicines, the total amount absorbed is similar and the timing change makes no practical difference. It becomes important for:

• medicines where small changes in blood level matter, sometimes called narrow therapeutic index drugs
• medicines that need to reach a peak quickly to work, such as some painkillers taken for acute pain
• medicines whose effectiveness depends on reliable absorption, such as oral contraceptives

The effect on stomach emptying tends to be greatest when starting treatment and after dose increases. That is why some advice, such as the contraception guidance for tirzepatide, focuses on those periods.

2. Effects on blood glucose

GLP-1 medicines increase insulin release when blood glucose is high and reduce glucagon. On their own they rarely cause hypoglycaemia. Added to medicines that lower glucose regardless of blood levels, such as insulin and sulfonylureas, the combined effect can push blood glucose too low.

3. Knock-on effects of side effects and weight loss

Vomiting, diarrhoea and reduced eating and drinking can lead to dehydration. That affects medicines cleared by the kidneys and those that themselves affect the kidneys. Weight loss also changes how much of some medicines you need, for example blood pressure tablets.

Interactions at a glance

This table is a general guide. The SmPC for each medicine, available on medicines.org.uk, contains the full interaction information, and your prescriber and pharmacist will apply it to you.

The contraceptive pill

This is the interaction most people ask about, and it is covered in depth in our guide to GLP-1s and contraception.

In brief, in June 2025 the MHRA issued guidance reminding people that GLP-1 medicines should not be used in pregnancy and highlighting that tirzepatide may reduce the effectiveness of oral contraceptives in people with overweight or obesity. Its advice for people taking tirzepatide who use oral contraception is to switch to a non-oral method, such as a coil or implant, or to add a barrier method such as condoms, for four weeks after starting and for four weeks after each dose increase.

For semaglutide and liraglutide, the SmPCs have not shown the same clinically relevant effect on oral contraceptives. However, vomiting or severe diarrhoea with any medicine can reduce how well the pill works, and the usual missed-pill rules may apply. Contraception services and pharmacists can advise.

The MHRA also advises that people should stop GLP-1 medicines before trying to conceive, with the length of the gap depending on the medicine, as set out in the SmPC. See our guide to GLP-1 medication, pregnancy and fertility.

Diabetes medicines

Insulin and sulfonylureas

The SmPCs for Mounjaro and Wegovy state that the risk of hypoglycaemia is increased when they are used with insulin or a sulfonylurea, and that a dose reduction of those medicines may be considered. In practice, your diabetes clinician may:

• reduce or stop a sulfonylurea when the GLP-1 medicine starts
• reduce your insulin dose, particularly as the GLP-1 dose increases
• ask you to check your blood glucose more often for a while

Symptoms of hypoglycaemia include shakiness, sweating, hunger, palpitations, irritability, blurred vision and confusion. If you take insulin or a sulfonylurea, make sure you know how to recognise and treat a low.

Metformin and SGLT2 inhibitors

Metformin and SGLT2 inhibitors (such as dapagliflozin and empagliflozin) do not cause hypoglycaemia on their own and are commonly used alongside GLP-1 medicines. Both can, however, be affected by dehydration. Metformin can cause stomach upset, which may add to GLP-1 side effects. SGLT2 inhibitors carry a rare risk of ketoacidosis, particularly when eating very little or during illness. Diabetes teams often give "sick day rules" for these medicines.

DPP-4 inhibitors and other GLP-1 medicines

DPP-4 inhibitors (gliptins) work by boosting the body's own GLP-1. Combining them with a GLP-1 receptor agonist adds little benefit, and they are usually stopped. Two GLP-1 receptor agonists should never be taken together. That includes combinations such as Ozempic with Wegovy, or Rybelsus with Mounjaro. If you are switching from one to another, your prescriber will tell you when to start the new medicine. Our guides to switching from Wegovy to Mounjaro and switching from an injection to a tablet explain how this is usually managed.

For more on the overlap between weight management and diabetes treatment, see our guide to GLP-1 medication and type 2 diabetes.

Warfarin and other narrow-margin medicines

Warfarin's effect is measured by the INR blood test, and small changes in absorption can shift it. The semaglutide SmPC recommends frequent INR monitoring when semaglutide is started in people taking warfarin or other coumarin derivatives. The tirzepatide SmPC similarly advises monitoring for medicines with a narrow therapeutic index when starting treatment. Changes in diet and weight can also affect the INR.

Other medicines where your prescriber may want closer monitoring include:

• some anti-epilepsy medicines
• lithium
• digoxin
• ciclosporin and tacrolimus after transplant

This does not mean these medicines cannot be combined with a GLP-1 medicine. It means the combination should be planned and monitored.

Newer anticoagulants such as apixaban, rivaroxaban and edoxaban do not need INR monitoring, but tell your prescriber you take them.

Thyroid medicines

Levothyroxine is commonly taken on an empty stomach, and its absorption is sensitive to food and other medicines.

For oral semaglutide, the Rybelsus SmPC reports that overall exposure to thyroxine increased by about a third in a study when given with oral semaglutide, and advises that monitoring of thyroid parameters should be considered. The Wegovy tablet contains the same active ingredient, so similar considerations are likely, but check its own SmPC and patient leaflet.

Timing is a practical challenge, because oral semaglutide also has to be taken on an empty stomach, with only a small amount of water, and with a wait before eating, drinking or taking other tablets. Your prescriber or pharmacist can help you work out a routine. Thyroid function tests may be repeated after starting.

For injectable GLP-1 medicines, the effect on levothyroxine is less direct, but weight loss can alter thyroid hormone requirements over time, so periodic checks are reasonable.

A separate point: the SmPCs mention a personal or family history of a rare type of thyroid cancer (medullary thyroid carcinoma) as a reason for caution. That relates to safety, not to an interaction, and is covered in our guide to who should not take GLP-1s.

Oral GLP-1 tablets: special rules

Two GLP-1 tablets are now licensed for weight management in the UK. Their interaction profiles differ, and both are newer in the UK than the injections.

Oral semaglutide (Wegovy tablets)

The MHRA approved oral semaglutide for weight management in June 2026. Semaglutide is a peptide, and to be absorbed from the stomach it must be taken under strict conditions. According to the UK approval announcement, it is taken on an empty stomach with a sip of water, after an overnight fast, and food and drink should be avoided for 30 minutes afterwards. Other oral medicines taken in that window may reduce semaglutide absorption, and semaglutide may in turn affect them. Check the current SmPC and patient leaflet for exact instructions.

Foundayo (orforglipron)

Foundayo was approved by the MHRA in August 2026. It is a non-peptide, small-molecule GLP-1 receptor agonist that can be taken at any time of day, with or without food. Because it is a different type of molecule, it may be processed by the body differently from peptide GLP-1 medicines, which could bring different interactions with medicines that affect liver enzymes. At the time of writing, UK real-world experience is limited. Rely on the current SmPC and your prescriber or pharmacist for interaction advice.

Our oral GLP-1 guide and our comparison of oral and injectable GLP-1s cover the tablets in more detail.

Blood pressure and heart medicines

As weight falls, blood pressure often falls too. That can be a welcome effect, but if you take blood pressure tablets, the combination can cause dizziness, especially on standing. Dehydration from vomiting or diarrhoea makes this more likely. Your GP may review and reduce doses over time.

Some heart rate effects are also relevant. GLP-1 medicines can cause a small increase in resting heart rate. If you take medicines that affect heart rhythm, your cardiologist or GP should know you have started treatment.

Kidney-sensitive medicines and dehydration

The SmPCs warn that fluid loss from gastrointestinal side effects can lead to dehydration and, in some cases, a decline in kidney function. Several common medicines can add to this risk:

• diuretics (water tablets) such as furosemide or bendroflumethiazide
• ACE inhibitors (such as ramipril) and angiotensin receptor blockers (such as losartan)
• non-steroidal anti-inflammatory painkillers (NSAIDs) such as ibuprofen and naproxen
• metformin and SGLT2 inhibitors, during dehydration

If you develop significant vomiting or diarrhoea, your prescriber may advise temporarily pausing some of these. That advice must come from a clinician who knows your history. Our hydration guide has practical tips.

Painkillers and other everyday medicines

• Paracetamol is generally considered suitable alongside GLP-1 medicines at the usual doses. Absorption may be slower, so relief may take a little longer.
• Ibuprofen and other NSAIDs can irritate the stomach and affect the kidneys, particularly when dehydrated. Ask a pharmacist.
• Opioid painkillers such as codeine slow the gut and can worsen constipation and nausea.
• Antacids and proton pump inhibitors may help with reflux, which is a common side effect. Our guide to reflux, burping and indigestion discusses options.
• Laxatives and anti-diarrhoea medicines can help manage side effects, used as directed. Prolonged or severe symptoms need review.
• Anti-sickness medicines may be prescribed for troublesome nausea. Some interact with other medicines, so these are a prescriber decision.

Supplements and herbal products

There is little interaction data for most supplements. Points worth knowing:

• Fibre supplements such as psyllium may help constipation but can increase fullness and bloating.
• Iron, calcium and magnesium can affect absorption of some medicines, including levothyroxine, if taken at the same time.
• St John's wort interacts with many medicines, including contraceptives, and could plausibly affect newer small-molecule medicines.
• Products marketed as "natural GLP-1" boosters are not equivalent to licensed medicines.

Our guide to vitamins and supplements on GLP-1 treatment covers which supplements may be considered.

Alcohol and recreational drugs

Alcohol is not listed as a direct pharmacological interaction in the SmPCs, but it matters in practice. It can worsen nausea and reflux, contributes to dehydration, adds calories, and in people taking insulin or sulfonylureas can contribute to hypoglycaemia. Heavy drinking is a risk factor for pancreatitis, which is a rare but serious side effect of GLP-1 medicines. See our guide to Mounjaro and alcohol.

If you use recreational drugs, telling your prescriber in confidence helps them give safer advice.

Interactions with procedures, not just medicines

GLP-1 medicines also interact with anaesthesia and sedation, because food may stay in the stomach longer. The MHRA issued a Drug Safety Update on this in January 2025. If you have surgery, an endoscopy or sedation planned, read our guide to GLP-1 medication before surgery and anaesthesia.

When interactions matter most

Interactions are not constant throughout treatment. They tend to matter most at points of change.

If you are treated privately and also see your GP for other conditions, these transition points are when communication between the two matters most. Letting your GP know when you start, change dose or stop helps them adjust your other medicines safely.

Getting your interactions checked

Whether you are prescribed through the NHS or a private provider, a safe consultation should include a medication review. A regulated online provider should ask about all your current medicines and may ask for your GP's details so information can be shared. If a service does not ask, treat that as a warning sign. Our guide to getting weight loss injections online safely explains what to look for, and you can see how regulated services compare on our provider comparison.

When you speak to your prescriber or pharmacist, it helps to have:

• a list of all prescribed medicines, with doses
• any over-the-counter medicines, supplements and herbal remedies
• your contraception method, if relevant
• any recent blood test results, such as kidney function or INR

For related reading, browse our safety guides, or visit our hubs for weight loss injections and oral GLP-1 tablets.

When to speak to a professional

Speak to your prescriber, GP or pharmacist:

• before starting a GLP-1 medicine, with a full list of your other medicines
• before starting any new prescribed, over-the-counter or herbal medicine
• if you use oral contraception and are starting or increasing tirzepatide
• if you take insulin or a sulfonylurea and notice lower blood glucose readings
• if you take warfarin and your INR changes
• if you feel dizzy or light-headed, especially when standing

Seek urgent medical help (call NHS 111, or 999 in an emergency) if you have:

• severe, persistent abdominal pain, which may spread to your back, with or without vomiting
• symptoms of severe hypoglycaemia, such as confusion, drowsiness or seizures, that do not resolve with treatment
• signs of severe dehydration, such as very little urine, confusion or fainting
• unusual bleeding or bruising if you take an anticoagulant
• signs of a severe allergic reaction, such as swelling of the face or throat, or difficulty breathing

Suspected side effects and interactions can be reported to the MHRA through the Yellow Card scheme.