Hi Ashley,
The duration of action of a drug is known as its half life. This is the period of time required for the concentration or amount of drug in your body to be reduced by exactly one-half. The half life is affected by many things such as age (how well the kidneys can excrete the drug), the type of drug it is or, sometimes, the drug may be translocated to another body fluid compartment and hang around there for a while. As far as I know, once the half life has been reached and you haven't taken any more of that med, the drug is considered to be ineffective or non-therapeutic. Drugs like ibuprofen that are cleared from the blood rapidly need to be given in regular doses to build up and maintain a high enough concentration in the blood to be therapeutically effective.
Scott
