Many words within Sonnet 147 are connected to similes and metaphors which can and do stand for something more profound as well as for what they are. Love is a disease to the speaker of this sonnet, so that means that Love at this point is not happy, but sickening. This sickness is not necessarily physical, although one could experience physical symptoms associated with the mental and emotion stress of this Love, but mostly it is just that--a mental and emotional sickness possibly based on a type of addiction. This addiction causes madness.
Next, the word "reason" is compared to the word "physician," but the speaker won't take the "prescription." The word "prescription" then really refers to reason's counsel to stop the illness-creating behavior. Because the speaker does not take his "medicine," or advice, then he becomes a madman who is beyond help. These are the words that mostly represent something other than what they stand for in the poem, but the device certainly helps to solidify the connection between being lovesick and refusing to listen to reason.