Several different factors can cause unintentional or unexplained weight gain. People gain weight when they consume more calories than they expend through activity. Common sources of unintentional weight gain include:

• Aging. As people grow older, their metabolism slows. This puts them at increased risk for weight gain, although very elderly people often lose weight because of reduced appetite.
  
  
• Certain endocrine disorders. These include hypothyroidism (underactive thyroid gland), Cushing's syndrome (overactive adrenal glands) and polycystic ovarian syndrome.
  
  
• Conditions involving edema (swelling due to accumulation of fluid). Examples include heart failure, pulmonary edema and kidney failure.
  
  
• Benign tumors and certain cancers, such as carcinoid tumors (tumors that release large amounts of serotonin and other substances) in the bile duct, pancreas, intestinal tract, lung or ovary.
  
  
• Pregnancy and menstruation. Women who are pregnant normally gain weight, which may be in addition to that of the developing fetus. Menstruation may also cause temporary weight gain in some women.
  
  
• Medications. These include corticosteroids, cannabinoids (marijuana-like drugs), antipsychotics and some drugs used to treat depression, bipolar disorder, diabetes (e.g., insulin, sulfonylureas), high blood pressure, seizures or heartburn. Patients are advised not to discontinue medications without talking to their physician. It is dangerous to halt some drugs abruptly or without a physician's supervision.
  
  
• Quitting smoking.