GLP-1 weight loss injections and other medications

The use of GLP-1/GIP weight management medications alongside other medications, including semaglutide (Wegovy®), liraglutide (Saxenda®) and tirzepatide (Mounjaro®).

What GLP-1 medications are and how they work

GLP-1 receptor agonists mimic the action of glucagon-like peptide-1, a natural hormone released by the intestine after eating.

They support weight loss through several mechanisms, including:

• Increasing insulin release when glucose is high

They encourage the pancreas to release insulin, mainly when blood sugar is elevated. This helps lower glucose after meals.

• Reducing glucagon release

Glucagon is a hormone that tells the liver to release stored sugar. GLP-1 medicines reduce glucagon, so the liver releases less glucose (especially after eating).

• Slowing gastric emptying, allowing food to remain in the stomach for longer

These medicines can slow how quickly food and drink leave the stomach and move into the small intestine. This often reduces post-meal sugar spikes and contributes to feeling full.

• Increasing satiety and increasing feelings of fullness after meals

They act on appetite centres in the brain, often leading to eating less and weight loss.

Together, these effects help reduce calorie intake and support sustainable weight loss when combined with healthy lifestyle changes.

Tirzeptide (Mounjaro) also acts on other hormone pathways (such as GIP) in addition to GLP-1, but the practical effects described above, especially appetite change and slowed gastric emptying, are still key for understanding medication interactions.

Why GLP-1 medicines can affect other medicines

The main reason GLP-1 medicines can change how other drugs behave is that they slow stomach emptying, which can change the timing and sometimes the amount of another medicine absorbed into the bloodstream.

This effect is most relevant for oral medicines (tablets, capsules, liquids) because those medicines usually need to move from the stomach into the intestines before they are absorbed efficiently. 

What “slowed stomach emptying” can do

As medicine (and food) sits in the stomach longer, this may:

• Delay the onset of effect of another medication (it can take longer to “kick in”)
• Change peak levels (sometimes the peak concentration is lower and later)
• In certain situations, there are increased stomach-related side effects of either the GLP-1 medicine or the other drug (nausea, heartburn, bloating)
• In people who develop significant vomiting or diarrhoea, it may also reduce how much of an oral medicine is absorbed simply because it is not retained long enough

Importantly: not every medication is affected, and many people take GLP-1 medicines safely with other drugs. However, the risk is higher for medications where timing matters, where a specific blood level must be maintained, or where there is a narrow safety margin.

Examples of medications where timing/absorption changes can matter

Below are categories often discussed with GLP-1 therapy. This is not a complete list, but it covers the major “watch-outs”.

• Medicines that need a fast onset – These may feel like they work more slowly:

• Pain relief taken as needed (e.g., paracetamol/acetaminophen, ibuprofen, some migraine tablets)

If absorption is delayed, relief may take longer. Some people notice they need to take these a

bit earlier (only under advice) or use an alternative formulation where appropriate.

• Rescue medicines that rely on quick action (certain anti-nausea tablets, or “as needed” anxiety medicines)

Timing may be altered, particularly early in GLP-1 dose escalation when gastric slowing and nausea is stronger.

• Medicines where small changes in level can be important (“narrow therapeutic index”) – These are not automatically unsafe with GLP-1 medicines, but they deserve extra care:

• Warfarin (and sometimes other anticoagulants depending on situation)

Weight loss, diet changes, vomiting/diarrhoea, and altered absorption can affect INR stability.

More INR checks may be needed when starting or changing GLP-1 doses.

• Thyroid hormone (levothyroxine)

Levothyroxine absorption is sensitive to timing and stomach/intestinal conditions. If

appetite, meal timing, or stomach emptying changes, thyroid levels may drift. Monitoring

(TSH) may be recommended after dose changes or significant weight loss.

• Certain anti-seizure medicines

Seizure control can be sensitive to absorption changes and to missed doses (for example if

vomiting occurs). Clinical monitoring is important.

• Immunosuppressants (e.g., tacrolimus/cyclosporine in transplant patients)

These drugs require stable levels; any change that affects absorption can matter. Specialist

oversight is essential.

• Diabetes medicines (risk of low blood sugar) – GLP-1 medicines on their own have a low risk of hypoglycaemia, but they can increase the risk if used with drugs that can cause hypos, especially:

• Insulin
• Sulfonylureas (e.g., gliclazide, glipizide)

Often, healthcare professionals will reduce insulin or sulfonylurea doses when GLP-1 therapy starts, and advise closer glucose monitoring, especially during the first weeks and after each dose increase.

• Blood pressure medicines and diuretics (dehydration risk) – If GLP-1 side effects include vomiting, diarrhoea, or poor intake, you may become dehydrated. Dehydration can:

• Lower blood pressure too much if you take antihypertensives (medication that lowers your blood pressure)
• Increase kidney strain with diuretics
• Increase risk of kidney injury, especially if also taking certain painkillers (NSAIDs)

In people at risk, healthcare professionals sometimes discuss “sick day rules” (what to pause during significant vomiting/diarrhoea), depending on the overall medication list and health conditions.

• Oral contraceptives

If vomiting or severe diarrhoea occurs, pill absorption may be reduced, which can decrease contraceptive reliability. Some GLP-1 medicines also have specific guidance about timing with oral contraceptives (particularly around treatment initiation and dose escalation). In practice, clinicians may recommend:

• Taking the pill at a consistent time,
• Using backup contraception during high-risk periods (vomiting/diarrhoea),
• Or considering a non-oral option if GI side effects are persistent.

• Medicines that irritate the stomach or require food timing – GLP-1 medicines can cause nausea/reflux, so medications that already irritate the stomach may feel worse:

• NSAIDs (ibuprofen, naproxen)
• Iron tablets
• Some antibiotics
• Bisphosphonates (e.g., alendronate)

Also, if a medicine must be taken with food or on an empty stomach, changes in appetite and meal timing can complicate adherence and may require a plan.

How GLP-1 medicines can increase side effects of other medicines

There are a few common mechanisms:

• Overlapping gastrointestinal side effects

GLP-1 medicines commonly cause nausea, early fullness, bloating, reflux, constipation or diarrhoea. If another medicine also causes GI upset (iron, metformin, some antibiotics, opioids, NSAIDs), the combined effect can feel stronger.

• Delayed stomach emptying can increase reflux/heartburn

If stomach contents stay longer, reflux may worsen, which can make certain tablets harder to tolerate.

• Vomiting/diarrhoea can lead to missed or poorly absorbed doses

This can reduce effectiveness of many oral medicines and may create fluctuations in control (blood pressure, seizures, thyroid symptoms, mood symptoms, etc.).

• Weight loss and dietary changes can shift medication needs

As weight drops and intake changes, doses for blood pressure medicines, diabetes medicines, and sometimes lipid or thyroid management may need reassessment.

When the interaction risk is highest

From what we understand, the interaction risk is highest:

• Early in treatment and during dose increases, when nausea and gastric slowing can be more pronounced.
• When a person has significant vomiting/diarrhoea, poor appetite, or dehydration.
• When taking medicines that require precise levels or fast action.
• In individuals who already have gastroparesis or other gastrointestinal motility issues.

Practical safety steps to be considered

Please keep the following guidance in mind:

1. Monitor how well your usual medicines are working

• If you notice reduced symptom control from any regular medication (for example, blood-pressure tablets, reflux medication, pain relief, or mental-health medicines), contact your GP or prescribing clinician.
• Do not stop or change your medication without medical advice.

2. Watch for increased side effects from other medicines

• If you experience new or worsening side effects from medications you were previously tolerating well, this may be due to altered absorption.
• Examples include dizziness, nausea, fatigue, palpitations, or stomach upset.
• If this occurs, seek advice from your GP or pharmacist.

3. Be extra cautious with medicines that require precise dosing

Some medicines are more sensitive to changes in absorption, including:

• Thyroid medication (e.g. levothyroxine)
• Diabetes medications (especially insulin or sulfonylureas)
• Blood-pressure medications
• Epilepsy or heart-rhythm medications

If you are taking any of these, regular monitoring may be required and your GP should be informed that you are using a GLP-1 medication.

4. Diabetes medications may need review

• GLP-1 treatments can lower blood glucose levels.
• If you are taking other diabetes medications, you may be at increased risk of hypoglycaemia (low blood sugar).
• Symptoms include sweating, shakiness, dizziness, confusion, or palpitations.
• If this happens, seek medical advice promptly.

5. Take oral medicines consistently

• Take your regular medications at the same time each day, unless advised otherwise.
• If stomach upset affects your ability to take tablets, do not skip doses – contact your GP or pharmacist for advice.

6. Inform all healthcare professionals

• Always tell your GP, pharmacist, dentist, or hospital team that you are using a GLP-1 medication.
• This helps ensure safe prescribing and appropriate monitoring.

7. Seek medical advice urgently if you experience:

• Persistent vomiting or inability to keep medicines down
• Severe abdominal pain
• Signs of low blood sugar
• Sudden worsening of symptoms from another medical condition

Summary statement

In summary, GLP-1 medicines help blood sugar and weight primarily by enhancing glucose-dependent insulin release, reducing glucagon, increasing satiety, and slowing gastric emptying. The slowing of gastric emptying and potential for gastrointestinal side effects can alter the timing and reliability of absorption of some oral medicines, which may delay their effect or intensify side effects, particularly during initiation and dose escalation, and especially for medications where precise levels or rapid onset are important.

Important Safety Advice

It is important to use GLP-1 medications safely and in line with clinical guidance.

Maintaining open communication with your provider is key to ensuring safe and effective treatment.

Remember:

• Do not ignore persistent or worsening symptoms
• Do not increase or stop your medication without advice
• Ensure your GP is informed of any new or unusual symptoms

Additional support

If you are experiencing side effects or have any questions about your treatment, our clinical team is here to help.

If your symptoms are severe, worsening, or you feel they require urgent medical attention, please seek immediate advice from your GP, NHS 111, or attend your nearest A&E department if appropriate.

For all other queries:

• Book a free video consultation with one of our trained clinicians through our website. Appointments are available throughout the week, including daytime and evening sessions.
• Send us your question online using our form: https://forms.medicinemarketplace.com/251345635207050 A member of our clinical team will respond by email.
• Speak to us by phone by calling 0330 175 5747, and selecting option 3.

We are here to answer your questions and provide support throughout your treatment journey.

Written by: Uzma Dala MPharm (2238839)

Approved by: Shameela Adam MPharm (2070113)