Mercaptopurine versus placebo to prevent recurrence of Crohn's disease after surgical resection (TOPPIC): a multicentre, double-blind, randomised controlled trial di 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