An increase in body fat results from a higher energy intake than expenditure. However, many factors mediate the food intake and energy balance and weight gain is not a simple calculation of intake minus expenditure. Several gut peptides, including leptin and ghrelin, are involved in regulation of hunger and satiety. Also, with age, the decline in reproductive hormones predisposes to weight gain.

A genetic predisposition coupled with a lifestyle that promotes unhealthy diets and lack of physical activity is the most common cause of obesity. Much less commonly, secondary causes such as hypothyroidism, Cushing’s syndrome, and polycystic ovarian syndrome cause weight gain.

In patients with serious mental illness, psychotropic medications contribute significantly to weight gain. Socioeconomic factors and unhealthy lifestyle habits are additional factors.

Many psychotropic agents cause weight gain as a side effect. Many mechanisms are thought to contribute to increased appetite, including central histamine (H1) blockade, serotonin (5HT2c) blockade, and dopamine (D2) blockade.

Most antipsychotics induce some degree of weight gain. Weight gain is more pronounced in medication-naïve patients. For instance, haloperidol, thought to be relatively weight neutral in patients with chronic illness, causes weight gain when used in medication-naïve patients [2]. Clozapine and olanzapine carry highest risk while ziprasidone carries the least risk of weight gain [2, 3]. Low-potency typical antipsychotics like chlorpromazine carry a significant risk of weight gain . Weight increases exponentially in the first months of antipsychotic treatment. In clinical experience, the maximal gain appears to be in the first 1–2 years though it can continue in later years also. There is no consistent evidence of a relationship between antipsychotic dose and risk of weight gain.

Antidepressants also can cause weight gain. Notable examples are mirtazapine and tricyclic antidepressants (TCAs) [4]. Evidence on extent of weight gain with selective serotonin reuptake inhibitors (SSRIs) and serotonin norepinephrine reuptake inhibitors (SNRIs) is variable across studies [4, 5]. Mood stabilizers such as lithium and valproate also cause weight gain though to a lesser extent than antipsychotics. Antihistamines when used consistently also can induce weight gain.

The following table categorizes psychotropic medications into approximate risk categories.

Besides the psychological effects and stigma of increased weight, the clinical manifestations of obesity result from its medical complications. Major complications include diabetes mellitus, dyslipidemia, coronary artery disease, and obstructive sleep apnea. Obesity also is a risk factor for liver disease, gall stone formation, osteoarthritis, fractures, skin infections, and some malignancies.

BMI is the first measure to determine unhealthy weight. It is calculated as body weight (kg)/height (meters) squared. Standard cutoff values may not be reliable in elderly, those with increased muscle mass, and in certain races. See table for weight and BMI categories.