It is a common complication of diabetes. The most common cause is poorly controlled blood sugar over time. Diabetic nerve pain can take years to develop. In the early stages, you may have no signs at all, and then only start to feel a tingling or numbness in your feet. As it progresses, you may also feel the pain in your hands and it is often worse at night.
Lastly, the aging process affects how drugs are excreted (eliminated) from the body. The elderly have a decrease in kidney function and blood flow to the kidneys. Due to this decrease, it is common to decrease the dose of drugs that are eliminated by the kidneys.
As a person ages, several physiological variables change. For instance, kidney function decreases, so drugs that are cleared by the kidneys will stay around longer. Body composition changes- you get more fat and less relative body water. Whether the drug is fat or water soluble then is also a factor. Lastly, your liver function may also decrease as you get older. The liver is so important in drug metabolism- both for the activation of some compounds and for the detoxification of others.