Antidepressants are widely prescribed, but the relationship between their use and weight gain is a common concern. Reports suggest that a significant percentage of users experience weight gain, affecting up to 55–65% of patients in some studies. This side effect is a major reason why some individuals stop taking their medication, potentially leading to a relapse of their original symptoms. Clarifying the actual mechanisms behind this weight change is important, as the perception of a “slowed metabolism” often oversimplifies the complex interaction between the drug, the brain, and the body’s energy balance.
Understanding Metabolic Rate and Weight
Metabolism is the set of chemical processes that occur within the body to maintain life, primarily converting food into usable energy. To understand how antidepressants might influence weight, it is helpful to define the body’s energy expenditure. The Basal Metabolic Rate (BMR) is the minimum amount of energy the body requires to keep its basic functions running while at complete rest, such as breathing and blood circulation.
The BMR typically accounts for 60% to 70% of the total energy the body uses each day. The remainder of the body’s Total Energy Expenditure (TEE) is composed of the energy needed for physical activity and the Thermic Effect of Food (TEF), which is the energy used to digest and process nutrients. Weight management requires balancing the calories consumed with the TEE; weight gain occurs when consumption consistently exceeds energy expenditure.
Direct Impact on Basal Metabolic Rate
The core question of whether antidepressants slow the BMR has a nuanced answer, as a dramatic decrease in the body’s baseline calorie-burning rate is not the primary cause of weight gain. Some scientific literature suggests that certain antidepressants may cause a subtle reduction in the Resting Metabolic Rate (RMR), which is closely related to BMR. This potential decrease in RMR might promote weight gain even if caloric intake remains unchanged.
Some older medications, such as tricyclic antidepressants, have been linked to increased energy efficiency, potentially allowing the body to function on fewer calories. This shift is sometimes attributed to minor changes in energy regulation, such as effects on thermogenesis, the body’s process of producing heat and expending energy. However, studies on specific drugs, like the SSRI fluoxetine, show that while it can lead to weight loss, it does not necessarily stimulate metabolism or increase the measured RMR. This suggests that while direct metabolic changes may play a small role, the effect is often minor and highly variable among individuals.
Indirect Mechanisms of Weight Change
The weight changes associated with antidepressant use are more commonly due to indirect mechanisms that affect the input and output sides of the energy balance equation. Many antidepressants interfere with neurotransmitters like serotonin and histamine, which regulate feelings of hunger and fullness. This interference can disrupt natural appetite signals, leading to increased cravings, particularly for carbohydrate-rich foods, or a reduced sense of satiety after eating.
The improvement in mood that accompanies successful treatment can restore a person’s appetite if it was suppressed by depression, naturally leading to weight gain as they return to normal eating patterns. Beyond appetite changes, many antidepressants cause side effects like sedation or fatigue, which decrease physical activity. This reduction in daily movement, known as Non-Exercise Activity Thermogenesis (NEAT), significantly lowers the Total Energy Expenditure (TEE) without affecting the BMR.
Antidepressants can also trigger hormonal shifts that influence how the body stores fat. Some medications have been linked to changes in the levels of hormones like leptin and ghrelin, which regulate long-term energy balance and hunger. There is also evidence that certain antidepressants can reduce insulin sensitivity, which affects how the body manages glucose and promotes fat storage. These neurochemical and hormonal changes are significant contributors to weight gain that are often mistakenly interpreted as a fundamental slowing of metabolism.
Varying Effects Across Antidepressant Classes
The risk of weight change is highly dependent on the specific class of antidepressant medication being used. Tricyclic Antidepressants (TCAs) and Monoamine Oxidase Inhibitors (MAOIs) are associated with the most substantial weight increases. The atypical antidepressant mirtazapine is also implicated in significant weight gain, largely due to its potent effect on blocking histamine receptors, which stimulates appetite.
Selective Serotonin Reuptake Inhibitors (SSRIs) can cause weight gain, especially with long-term use, though the risk is lower than with TCAs or mirtazapine. Paroxetine is often cited as having a higher propensity for weight gain compared to other SSRIs. Conversely, some medications, such as bupropion (a norepinephrine-dopamine reuptake inhibitor), are frequently associated with weight neutrality or even modest weight loss. This variation underscores that the weight effect is a side effect tied to the drug’s specific chemical structure and its interaction with various receptors in the body.