Mercaptopurine versus placebo to prevent recurrence of Crohn's disease after surgical resection (TOPPIC): a multicentre, double-blind, randomised controlled trial nork Mowat, C, Arnott, I, Cahill, A, Smith, M, Ahmad, T, Subramanian, S, Travis, S, Morris, J, Hamlin, J, Dhar, A, Nwokolo, C, Edwards, C, Creed, T, Bloom, S, Yousif, M, Thomas, L, Campbell, S, Lewis, S, Sebastian, S, Sen, S, Lal, S, Hawkey, C, Murray, C, Cummings, F, Goh, J, Lindsay, J, Arebi, N, Potts, L, McKinley, A, Thomson, J, Todd, J, Collie, M, Dunlop, M, Mowat, A, Gaya, D, Winter, J, Naismith, G, Ennis, H, Keerie, C, Lewis, S, Prescott, R, Kennedy, N, Satsangi, J, TOPPIC Study Group