Sudden heart failure or other problems can develop when the body is worked harder. Loss of reserve also makes it harder to restore equilibrium in the body. Drugs are detoxified at a slower rate. So, lower doses of medications may be needed, and side effects become more common. Medications side effects can mimic the symptoms of many diseases, so it is easy to mistake a drug reaction for an illness.
Some medications have entirely different side effects in the elderly than in younger people.
No one really knows how and why people change as they get older. Other theories view aging as a predetermined, genetically-controlled process. However, no theory sufficiently explains all the changes of the aging process. Aging is a complex and varied process that varies in how it affects different people and organs. Things that affect the aging process include heredity, environment, cultural influences, diet, exercise and leisure, past illnesses, and many other factors.