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