I don't know of anyone who uses any medicated feed to get a pullet to start laying. There occasionally is someone here who uses cayenne pepper added to feed to start laying, but I honestly don't know if the cayenne makes any difference, or if the birds were just ready to lay anyway.
If you just allow them to lay when they are ready, you won't have to worry about a withdrawal period. I can't imagine waiting for those beautiful first eggs, and then having to throw them away because I was so eager that I used medication to try to make it happen faster. That seems like a terrible waste to me, but you have to decide what you are comfortable with.
Many birds are ready to lay by 20 or so weeks, but it depends on the particular bird. We have had some that didn't start until they were over 10 months. We just waited patiently, and they all laid eventually. Our new set is about ready to lay, too. Interestingly enough, three of the birds are two weeks younger than the others, and it is two of those younger birds that really show all the signs of being ready. They all just have their own body rhythm.