by Sergio Silva, M.D.

Created on: August 20, 2009   Last Updated: August 21, 2009

One of the most alarming and universally unacceptable of medication side effects is that of permanent weight gain, and it is not an uncommon one. This article focuses on weight gain as a side effect of psychotropic medication, which falls under the area of my expertise, but the general principles are applicable to weight gain from any medication or substance-including, for example, the recreational drug marijuana.

Weight changes due to the routine ingestion of a chemical compound, be it a prescription medication, an over-the-counter preparation, or some other substance, could be due to three primary mechanisms.

1. Water retention or loss. The composition of normal body tissues is largely aqueous; at least 70% of our body weight is attributable to the weight of water. Water weight is not, however, what people worry about when they think about weight change due to medication. Although the bathroom scale can register sometimes rather impressive shifts due to this mechanism, and bloating due to water retention can feel uncomfortable and make your clothes fit more snugly about the waist, this is a relatively modest effect that is seen mostly with prescription hormone therapies. It waxes and wanes depending on the dosing schedule and is not the kind of weight gain that involves an increase in the actual size of fat cells. Similarly, but in the opposite direction, diuretics which are used to lower blood pressure may also result in a rapid and pronounced weight change according to the scale, but no actual permanent change occurs with regard to a person's girth.

2. A change in metabolism. A few commonly prescribed medications directly impact a person's metabolic rate, increasing it (medications that significantly lower a person's metabolism, such as those used in anesthesia, are administered in highly controlled settings for brief periods of time and are not generally prescribed to outpatients). Thyroid hormone, for example, stimulates the metabolism and is prescribed in hypothyroid conditions and also as an adjunct in treating medication-resistant major depression. Drugs that increase sympathetic arousal-that is, stimulants-increase metabolism by virtue of the fact that they increase the activity of the central nervous system and stimulate the heart rate, both of which consume extra energy.

But, again, medication-induced weight change is not typically accomplished by this particular mechanism. Patients taking stimulants lose weight because

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