When it comes to medications, one size does not fit all. The genes that make you unique also may affect how you respond to the medicines you take. So, the standard dose that works for most people may not be adequate for you, or it could trigger potentially dangerous side effects. Add to the mix the age-related changes in your metabolism and its easy to understand why some prescriptions may work immediately and others may require some tweaking. "Its tricky, and drug metabolism can play a role in the dose of medications," says Mandy Leonard, R.Ph., Pharm.D., BCPS, manager of Cleveland Clinics Drug Information Service. "Sometimes you cant say one dose fits all. Every patient is a little different, so there could be some dose adjustments a physician would need to consider."