Possible causes of unexplained weight loss include changes to a person's sense of smell or taste, states Mayo Clinic. A number of diseases can also cause weight loss as a symptom, including heart failure, Parkinson's disease and cancer.
Medication can cause unexpected weight loss, according to Mayo Clinic, as can abuse of such substances as alcohol and cocaine. Depression and dementia might lead to weight loss, as can an overactive or underactive thyroid. Other potential causes include problems with the digestive tract, including peptic ulcers, Crohn's disease and ulcerative colitis. Dental problems are another potential cause, and patients with diabetic hyperglycemia might experience weight loss as a symptom.
People who lose 5 percent of their body weight or more without a clear cause within six to 12 months should see their doctors, explains Mayo Clinic. Older, frailer adults should speak with a doctor even if they lose less than five percent of their body weight in a short period of time. If a basic evaluation doesn't reveal any signs of other diseases, the next step is often to wait and look for any changes; full-body cancer screenings are not usually helpful unless there are other signs that the patient might have cancer. Doctors might ask patients to go on a special diet to regain weight.