After 40, men typically lose 3–8% of muscle mass per decade and testosterone declines by around 1% a year — both of which slow metabolism and make visceral fat more likely to accumulate around the middle. The treatments themselves (Mounjaro, Wegovy injection, the Wegovy pill) work the same way regardless of age, but success after 40 usually depends more on protecting muscle mass and staying consistent with resistance training and protein intake alongside treatment, since the natural age-related trend works against you on both fronts.
Metabolism Slows Gradually, Not Overnight
Basal metabolic rate — the energy your body burns at rest — declines gradually from your 30s onwards, driven largely by the loss of muscle mass described below. Since muscle burns more energy at rest than fat does, less muscle means a lower daily calorie burn even if your activity levels haven't changed, which is why the same diet and exercise routine that maintained your weight in your 30s can quietly stop working in your 40s.
Muscle Mass Declines Year on Year
Most men begin losing muscle mass gradually from their 30s, at a rate of around 3–8% per decade, a process that accelerates further after 60. This matters for weight management specifically because muscle tissue is metabolically active — it burns calories even when you're not exercising — so less muscle compounds the slowdown in metabolism described above.
Testosterone Begins a Slow, Steady Decline
Testosterone levels typically fall by around 1% a year from age 40 onwards. This is a normal part of ageing rather than a medical condition in itself — most men's levels remain within the normal range throughout their 40s and 50s, and only around 10–25% of older men have testosterone low enough to be classed as clinically significant. Since testosterone plays a role in maintaining muscle mass and influences where the body tends to store fat, this slow decline works in the same direction as the muscle loss and metabolic slowdown above, making visceral fat more likely to accumulate around the abdomen.
Worth knowing
These three changes are gradual trends, not a switch that flips at your 40th birthday, and they affect every man differently depending on genetics, activity levels and overall health. The point isn't that weight loss becomes impossible after 40 — it's that the strategy needs to account for a slower metabolism and a natural pull towards muscle loss, both of which respond well to resistance training and adequate protein intake.
Mounjaro, Wegovy injection and the Wegovy pill work the same way at any adult age — there's no age-specific dosing or reduced effectiveness reported in the trials for men in their 40s, 50s or 60s. What does matter more after 40 is protecting the muscle mass you have while losing fat, since some lean mass loss is a normal part of any significant weight loss, and men over 40 are already losing muscle at a background rate before treatment even starts.
| Mounjaro | Wegovy Injection | Wegovy Pill | |
|---|---|---|---|
| Active ingredient | Tirzepatide | Semaglutide | Semaglutide (oral) |
| How it's taken | Weekly injection | Weekly injection | Daily tablet |
| Dose range | 2.5mg – 15mg | 0.25mg – 2.4mg (7.2mg licensed) | 1.5mg – 25mg |
| UK approval | 2023 | 2022 (7.2mg: Jan 2026) | 11 June 2026 |
All three require a prescription and a pharmacist-reviewed consultation. For a full breakdown of how they compare, see our main guide to weight-loss treatments for men.
The steps below reflect standard, general lifestyle guidance for supporting muscle mass and metabolic health during weight loss at any age — they aren't medical advice specific to you, and any exercise or diet changes should take your existing health conditions into account.
Make resistance training non-negotiable
Two to three sessions a week — whether gym-based, bodyweight, or resistance bands — gives your body a reason to hold on to muscle during a calorie deficit, directly counteracting the age-related muscle loss described above.
Front-load protein at each meal
Adequate protein intake supports muscle protein synthesis and helps offset the natural decline in muscle mass. Good sources include lean meat, fish, eggs, dairy, and plant-based options like lentils and tofu.
Prioritise sleep and stress management
Poor sleep and chronic stress both raise cortisol, a hormone that can encourage fat storage around the abdomen and interfere with recovery from exercise — both particularly relevant given the natural shift towards visceral fat after 40.
Review progress by more than the scale
Waist circumference and how your clothes fit often tell a clearer story than body weight alone, especially in the first few months, since muscle-preserving weight loss can look different on the scale to rapid, less sustainable weight loss.
Eligibility is confirmed during your free online consultation, and there is no upper age limit written into the licensing for Mounjaro, Wegovy injection or the Wegovy pill. As a general guide you may be eligible if you have a BMI of 30 or above, or a BMI between 27 and 30 with at least one weight-related condition such as high blood pressure, high cholesterol or type 2 diabetes.
Important
Men over 40 are more likely to have existing health conditions or take other regular medication, both of which your prescriber needs to know about. Always disclose your full medical history, including any heart, kidney or thyroid conditions and any medicines or supplements you currently take, during your consultation.
Ready to Talk to a Pharmacist About Your Options?
Every treatment plan at Happy Pharmacy starts with a free online consultation, reviewed personally by our GPhC-registered pharmacy team, so you get a recommendation based on your own health history rather than a generic starting point.
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- Mayo Clinic. Male menopause: Myth or reality?
- ColumbiaDoctors. How Testosterone Changes Affect Men's Health as You Age.
- Harvard Health Publishing. Preserve your muscle mass.
- Prado CM, et al. Sarcopenia: a dive into metabolism to promote a multimodal, preventive, and regenerative approach. 2024.
- NHS England Digital. Health Survey for England, 2024: Adults' overweight and obesity.
- NICE. Obesity: identification, assessment and management guidance — waist circumference thresholds.
- Jastreboff AM, et al. Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1). N Engl J Med. 2022.