Estimates indicate that 14 million Americans have olfactory dysfunction. As with other senses, such as sight and hearing, olfaction frequently declines with age. Impaired olfaction can be a warning sign of Parkinson's disease, sometimes occurring before motor symptoms develop. It's also an initial symptom of Alzheimer's dementia (AD); the amyloid plaques and tangles characterizing AD invade the olfactory bulb and hippocampus early in its course, hampering odor identification. Olfactory dysfunction is associated with some serious problems, including inability to smell warning odors (fire, gas) and impaired ability to taste food. Standardized, validated methods are available to measure several different dimensions of olfactory function, including odor identification, discrimination, and threshold levels. Researchers are currently studying the unique olfactory deficits associated with different conditions in hopes of identifying new, noninvasive tools for early diagnosis and treatment. Drugs may cause or contribute to olfactory dysfunction, but it can be difficult to pinpoint offending medications.