Weight loss drugs and fasting: is it safe to restrict your diet?

Is it safe to take weight loss injections during Ramadan or while fasting intermittently? Learn what experts say and what precautions to take.



It's always a good idea to speak to your GP or medical team before making significant changes to your diet. They can offer tailored advice to help you stay safe and comfortable while fasting.

Remember: if you're a SemaPen subscriber, you can contact our support team at any time via WhatsApp. Here's the link: https://wa.me/message/F4UOIVRQPJDXP1

Stick to a strict schedule

Doctors recommend taking your weekly Wegovy or Mounjaro injection on the same day each week. This is to help maintain a stable and consistent level of the drug in your body.

While fasting, this consistency is especially important. Try to take your injection on the same day and at roughly the same time each week. It's OK to take your injection during a fasting period.

If you're fasting for Ramadan:  try taking your weekly dose around 30 minutes before your Iftar meal. This gives the appetite-suppressing effects time to kick in, and means you might be less likely to overeat when breaking your fast.

Be mindful of side effects

Fasting while taking Wegovy or Mounjaro might increase the risk of certain side effects, such as dizziness and weakness. Be sure to listen to your body and look out for signs of side effects like these. Your SemaPen team or GP can advise on how to proceed.

If you're fasting for dietary reasons, take it slow and steady

Remember that everyone's body works differently, and we all experience hunger in different ways. Some people find that the appetite-suppressing effects of weight loss injections make fasting feel easier. For others, however, it can increase the challenge.

If you're able to change your fasting schedule, it's worth experimenting to see what works best for you. Consider starting with a shorter fasting window (12 to 16 hours) and make note of how your body responds. You can then increase or decrease the window depending on how comfortable you feel.

Stay hydrated

Drinking plenty of fluids is crucial for our overall health and well-being. The UK government recommends drinking six to eight glasses of healthy, sugar-free fluids per day.²

But you should pay special attention to your hydration while fasting and using weight loss medications. This is because:

• Your body releases more water and salt in urine while fasting, especially during the first few days of a fast. This can lead to dehydration.
  
  
• Certain side effects of Wegovy and Mounjaro, such as vomiting and diarrhoea, can also increase dehydration.
  
  
• It can be harder to remember to stay properly hydrated when you're not eating regularly.
  
  

Look out for signs of dehydration. If you develop severe headaches, feel dizzy while standing or notice dark urine when you go to the toilet, seek immediate medical advice.

If you're fasting for Ramadan:  make sure to drink plenty of water when you're allowed to. Consider adding electrolytes to your water, too. These help maintain fluid balance in the body during long periods without food or drinks.

Don't go overboard with exercise

Fasting can affect energy levels, so you should take extra care when exercising. This is especially true for Ramadan fasts, where eating and drinking are restricted for long periods.

During fasts, focus on low-intensity activities such as light yoga, walking or dynamic stretching. And if you do want to engage in high-intensity exercise, try to schedule your workouts right before meals or during your eating window. This way, you can fuel up with calories soon after.

Again, your body should tell you what activity level is right for you. If you feel especially tired while exercising or experience symptoms like dizziness or heart palpitations, take it as a sign to slow down.

Eat the right food in the right portions

Eating a healthy, balanced diet is the single most important thing you can do to lose weight safely and sustainably. This means eating the right amounts of the right kinds of food.

Again, this is especially crucial while fasting. When you can eat, be sure to prioritise:

• Low-fat proteins, such as lean meats and fish, as well as eggs, beans and pulses. Protein helps keep us full and protects our muscles.
  
  
• Fibre, which helps stave off hunger and promotes digestive health. Good sources include fruits, vegetables and whole grains.
  
  
• Small portions of healthy but calorie-dense snacks. Think foods with healthy fats like nuts and avocados. These will give you a welcome energy boost when you break your fast.

Don't be tempted to overeat when you break your fast, either. That's because weight loss injections slow down the digestive system. Overeating could lead to digestive discomfort, so start with small portions and eat slowly.

Have expert support at hand

At SemaPen, we offer tailored guidance to all our Wegovy and Mounjaro patients. That's because we know that expert support is vital in achieving healthy, sustainable weight loss.

Our support helps our subscribers stay on track and stay motivated. But more than that, it means they can access real-time advice tailored to their health, their diet and their circumstances.

This is hugely beneficial if you're undergoing a significant lifestyle change, like a period of fasting. With SemaPen, you'll never go through your fast alone. Instead, you'll have a team of experts on hand to help keep you safe and comfortable throughout.

And you can find out today if you're eligible for our programmes. All you have to do is complete our simple digital consultation. We'll respond straight away with recommended treatment options for your unique needs.

Sources

1. Almaqhawi, A. et al. (2025) "Impact of Fasting on Physical Activity Motivation and Weight Reduction in Patients Administered Glucagon-Like Peptide-1 Agonists: A Qualitative Study" Patient Preference and Adherence, 19:19–28

https://doi.org/10.2147/PPA.S496613

2. NHS (2022) The Eatwell Guide. Retrieved from https://www.nhs.uk/live-well/eat-well/food-guidelines-and-food-labels/the-eatwell-guide/

This article was reviewed and approved by Lujain Alhassan, Bariatric Dietitian, and Sarah Abdula, Specialist Dietitian, on 5 January 2026.