Weight Gain: Causes, Health Risks & What You Can Do About It

Weight gain affects nearly two-thirds of UK adults and is associated with a substantially elevated risk of type 2 diabetes, cardiovascular disease, and multiple cancers. Understanding why weight gain happens — and what can be done about it — is central to making meaningful, sustainable change.

This guide covers the principal causes of weight gain, the associated health risks, effective lifestyle interventions, and when MHRA-approved prescription treatment may be appropriate.

Causes of Weight Gain

Weight gain results from a sustained calorie surplus — consuming more energy than the body expends. However, this simple equation conceals considerable biological complexity. The factors driving calorie intake and energy expenditure are diverse, and for many people, biological and hormonal factors make sustained weight loss extremely difficult without additional support.

Medications that can cause weight gain

Several commonly prescribed medications are associated with weight gain as a side effect. These include certain antidepressants (particularly tricyclics and SSRIs), corticosteroids used long-term, antipsychotic medications, and some treatments for type 2 diabetes (including sulfonylureas and insulin). It is important never to stop or change prescribed medication without consulting a GP, even if weight gain is suspected as a side effect.

Hormonal and metabolic conditions

Undiagnosed or undertreated hormonal conditions can make weight management significantly harder. These include:

• Hypothyroidism — an underactive thyroid slows metabolism, making weight gain more likely and weight loss more difficult
• Polycystic ovary syndrome (PCOS) — associated with insulin resistance and a tendency towards weight gain, particularly around the abdomen
• Cushing’s syndrome — excess cortisol production leads to characteristic weight gain, particularly centrally
• Insulin resistance — impairs the body’s ability to regulate blood glucose effectively, promoting fat storage

If weight gain persists despite genuine lifestyle change, a blood test with a GP can rule out underlying hormonal or metabolic contributors.

Genetic factors

Research consistently demonstrates that genetic factors account for 40–70% of BMI variability between individuals. Genetic predisposition does not mean weight cannot be managed — but it does help explain why some people find it significantly more difficult than others and why biological support (including medication, where appropriate) can be a legitimate and evidence-based choice.

Health Risks of Excess Weight

Lifestyle Changes That Work

For many adults, a combination of dietary modification, increased physical activity, improved sleep, and stress management is sufficient to achieve clinically meaningful weight loss. NICE guidance identifies the following as the evidence-based components of effective lifestyle-based weight management:

• Moderate calorie deficit — aim for approximately 500–600 kcal below daily maintenance energy requirements
• Reduce ultra-processed foods — replace with whole foods, lean protein, vegetables, and dietary fibre, which promote satiety and support a healthy gut microbiome
• Regular physical activity — NICE recommends at least 150 minutes of moderate-intensity activity per week; even brisk walking significantly improves metabolic health
• Improve sleep quality — consistent 7–9 hours of sleep supports healthy regulation of ghrelin and leptin, reducing appetite-driven overeating
• Manage stress — mindfulness, structured relaxation, therapy, and regular activity all reduce cortisol-driven eating behaviour
• Behavioural support — NICE consistently identifies behavioural interventions as improving long-term weight loss outcomes compared to dietary advice alone

When Medication Might Help

NICE guidance recommends considering prescription weight loss medication as an adjunct to lifestyle modification — not a replacement for it — for adults who meet clinical eligibility criteria and have been unable to achieve clinically meaningful weight loss through lifestyle changes alone.

Eligibility criteria (NICE CG189 and TA guidelines): a BMI of 30 or above, or a BMI of 27–29.9 with at least one weight-related health condition (such as type 2 diabetes, hypertension, or dyslipidaemia), and evidence of genuine prior lifestyle modification.