Weight Loss Injections and Your Other Medicines
Weight loss injections (such as semaglutide or liraglutide) are prescription medicines used to help people lose weight when lifestyle changes alone are not enough. These medicines work mainly by slowing down how quickly food leaves your stomach and by reducing appetite.
Because they slow digestion, they can also affect how quickly other medicines you take by mouth (oral medicines) are absorbed into your body. This may change how well those medicines work.
Common Weight Loss Injections
Semaglutide (brand name Wegovy, Ozempic)
Liraglutide (brand name Saxenda, Victoza)
Tirzepatide (brand name Mounjaro)
Why This Matters
When digestion is slowed, tablets or capsules can take longer to reach the part of your gut where they are absorbed. This may:
Delay the start of the medicine’s effect
Reduce how much medicine gets into your bloodstream
Occasionally increase side effects
Examples of Oral Medicines That May Be Affected
| Medicine type | Example medicines | Possible effect if taking with weight loss injections |
|---|---|---|
| Diabetes tablets | Metformin, gliclazide, dapagliflozin | Risk of low blood sugar (especially if on sulfonylureas like gliclazide); dose may need adjusting |
| Blood thinners | Warfarin | Changes in INR levels; closer blood tests may be needed |
| Thyroid tablets | Levothyroxine | May be absorbed more slowly; blood tests may need repeating after starting injections |
| Epilepsy medicines | Phenytoin, carbamazepine | May change blood levels; seizures could become more or less likely |
| Antibiotics | Some antibiotics work best at a steady level in the blood; delayed absorption could reduce effectiveness | |
| Oral contraceptives | Combined pill, mini-pill | Possible reduced absorption if you also have vomiting or diarrhoea from injections; use extra contraception if this happens |
What You Should Do
Tell your clinician about all the medicines you take before starting weight loss injections (including vitamins, herbal remedies, and over-the-counter medicines).
Do not change doses yourself. Your GP or pharmacist may need to adjust your doses or change the timing of your medicines.
Report side effects promptly, especially:
Unusual bruising or bleeding (blood thinners)
Symptoms of low blood sugar: sweating, shaking, hunger, dizziness (diabetes tablets or insulin)
Worsening control of your health condition
Extra contraception: If you have vomiting or diarrhoea for more than 24 hours while on weight loss injections, use condoms as well as your pill for the next 7 days.
Regular monitoring: Some medicines will need extra blood tests after you start weight loss injections.
When to Seek Urgent Help
Call 111 or seek immediate medical attention if you have:
Severe stomach pain with vomiting
Signs of low blood sugar that do not improve after eating or drinking
Signs of blood clots (swelling, redness, or pain in a leg; sudden shortness of breath)
Signs of an allergic reaction (swelling of the face/lips/tongue, difficulty breathing)
