You can develop side effects after taking a drug for years without any previous problems. Drug side effects usually occur within hours or days, but you could experience symptoms including dizziness, confusion, sleepiness, nausea, and falls even after taking a drug long-term. Age-related changes in your body metabolism, such as an increase in body fat and reduced kidney and liver function, may keep drugs in your system longer as you get older, raising the risk of side effects. Adding new drugs, over-the-counter remedies, or supplements to ones you already take could also cause interactions between medications, resulting in new side effects.
Source: Consumer Reports on Health March 2015