Dosage is important - think of it this way:
You go to thepub and neck 7 pints in an hour. Guessing you'll definitely feel the effect, right?
Now, go to the pub once a night for a week and have one pint a time. You'll feel no noticeable affect due to your body's ability to deal with the dose - the way it metabolises it.
Similarly, your body may metabolise a smaller dose quickly before it can have the desired effect, hence the need to take the dose as described in your prescription or pill bottle.
The other side of the story: Bear in mind that medicines are toxins too, so they need to hit the part they treat fast, then allow your body to get rid of them so you don't end up poisoned.