Why are there qid and tid treatments?

Answer 1

That's because every drugs has a certain therapeutic levels to be effective. Your body naturally starts to remove the drugs from the system the moment they enter the blood stream. Certain drugs can be removed quicker than others. So, in order to maintain the level in the blood, a drug has to be replaced at a regular interval to be effective. This interval is determined by the half life of the drug. That's the time it take for the concentration to reduce in half.

Keep in mind that this half life time is the average. In reality, everybody is different. The conditions of your liver, kidneys, as well as, other medications/chemicals/food can alter the half life.