Why Feel Tired on Mounjaro? 8 Overlooked Causes

Starting a new weight management programme can be incredibly exciting, but many patients find themselves feeling surprisingly tired on Mounjaro. While it is easy to assume that your exhaustion is a direct side effect of the medication itself, there are often other hidden factors at play. In this guide, our superintendent pharmacist, Zeinab Ardeshir, walks us through eight overlooked medical causes of persistent tiredness that you might experience during your journey to a healthier lifestyle.

Understanding Tiredness on Mounjaro

Tiredness is one of the most common reasons patients visit a healthcare professional. However, “tiredness” can mean very different things to different people. To address the root cause of why you feel tired on Mounjaro, we first need to categorise the type of exhaustion you are experiencing:

• Sleepiness: This is characterised by feeling drowsy, nodding off during the day, or waking up feeling unrefreshed despite having slept for several hours.
• Fatigue: A heavy, drained sensation that persists despite resting. This deep physical exhaustion is often linked to chronic conditions, hormonal shifts, or metabolic changes.
• Reduced exercise tolerance: Finding that you must slow down during physical activities that used to be manageable, such as walking up stairs that you used to manage without becoming breathless. This type of tiredness is more closely tied to heart or lung health.

By identifying which category your symptoms fall into, it becomes much easier to pinpoint the underlying medical causes and find an effective treatment plan.

8 Reasons You Feel Tired on Mounjaro

If you are on a weight loss journey, it is tempting to blame your weekly injection for your low energy. However, there are eight distinct, overlooked medical reasons that could be causing your fatigue.

1. Nutrient Deficiencies

This is one of the most common causes of low energy on this treatment. When using a GLP-1 receptor agonist, your appetite is significantly suppressed, meaning you eat much less. If your food intake drops, your intake of essential vitamins and minerals drops too. The four most critical nutrients tied to energy levels are iron, folate, vitamin B12, and vitamin D. Iron deficiency is particularly common in women’s health, and a lack of any of these four nutrients can quickly lead to anaemia and severe lethargy.

2. Thyroid Problems

The thyroid is a vital organ that regulates the speed of your metabolism. If your thyroid slows down (a condition known as hypothyroidism), it can cause extreme tiredness, constipation, and unexpected weight gain. Because thyroid hormone levels have a direct impact on your metabolic rate, keeping your thyroid healthy is an important part of your general health while taking a GLP-1 medication. It is highly recommended to have your thyroid levels checked before starting treatment, and again if you experience significant weight loss.

3. Sleep Apnoea

Sleep apnoea is a condition where you temporarily stop breathing for short periods during the night, forcing your brain to wake you up to take a deep breath. This severely disrupts your sleep quality, leaving you exhausted the next day. Sleep apnoea is closely linked to obesity. While losing weight can often help improve this condition, the transition period can leave you feeling incredibly drowsy.

4. Blood Sugar Fluctuations and Diabetes

Fatigue is a hallmark symptom of unstable blood glucose levels. Mounjaro is highly effective at managing blood sugar (HbA1c) levels for those living with type 2 diabetes. However, if you are pre-diabetic or transitioning to lower blood sugar levels, these metabolic shifts can temporarily drain your physical energy as your body adapts.

5. Heart and Lung Health

If your tiredness presents as breathlessness or a racing heart during minor physical exertion, it may be related to your heart or lung health. Conditions like Chronic Obstructive Pulmonary Disease (COPD) or underlying heart issues reduce your exercise tolerance. Carrying excess weight puts extra strain on these organs; as you lose weight, this strain will decrease, supporting your long-term cardiovascular health and healthy living goals.

6. Medication Side Effects

Many patients take other daily medications that list drowsiness as a primary side effect. Common culprits include antihistamines, codeine-based painkillers, gabapentin, certain blood pressure medications, and antidepressants. You should never stop taking prescribed medications without consulting a professional, but it is important to recognise if another drug is compounding your tiredness.

7. Mental Health and Burnout

Your psychological state has a profound effect on your physical stamina. Experiencing low mood, anxiety, depression, or professional burnout can leave you feeling completely devoid of energy. Addressing your mental health is a vital part of any weight management journey, as the brain and body must work in harmony.

8. Post-Viral Fatigue

Recovering from viral infections like influenza or long COVID can leave you with prolonged, chronic fatigue. This often manifests as a “crash” where you feel completely burnt out a day or two after experiencing physical or mental exertion. Pacing yourself is essential to allow your immune system to fully recover.

Managing Symptoms When You Are Tired on Mounjaro

If you’re experiencing this kind of fatigue, there are several practical lifestyle changes you can implement to support your body. Because your food intake is lower, focus on a high-protein diet and maintain excellent hydration throughout the day. This provides your muscles with the necessary fuel to prevent wastage and keep your metabolism active.

Additionally, avoid the temptation to rush your weight loss. Aiming for rapid, drastic weight drops can shock your system and deplete your energy reserves. A gradual, sustained weight loss is far healthier, allowing your brain and physical body to safely adjust to your changing shape.

When to Seek Medical Advice

While minor fatigue can occur as your body adapts to a calorie deficit, persistent exhaustion should not be ignored. You should speak to a medical professional if your tiredness lasts longer than six to eight weeks, or if it is actively getting worse.

Be sure to seek immediate clinical advice if your fatigue is accompanied by red-flag symptoms such as severe breathlessness, an irregular or pounding heartbeat, changes in your toilet patterns (such as extreme constipation or excessive urination), or sudden blurry vision. Most of the underlying causes of tiredness can be easily diagnosed with a routine blood test and managed with targeted supplements or minor lifestyle adjustments.

Disclaimer: This blog post is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional before making changes to your health routine.