This process may be used for medicines which you need one way or the other but your body does not accept it causing an allergic reaction.
For eg in Desferal, I had to take the medicine for the sake of survival but my body was not accepting the drug. The only way then is to force the body to accept it. One way of doing this is desentisization when the body is exposed to the medicine over a long period of time (8 to 10 hours) starting of with extremely low concentration levels and increasing the dosage after every 30 to 60 mins so that the body starts getting used to the drug. In the end of the process the allergic tendencies of the body for that particular drug fades away (not completely-benadryl is also used as a compliment to help the case) and the regular dosage may be administered for 2 or 3 straight nights as the body is desentisized.
This is a very long process and is used for exceptions only when there are severe allergies related. It is not regular practice.
Hope it's clear