Back in 1999 when I was put on 6MP my GI stated that he only allows his patients to stay on it for a max of 5 years. It's supposed to replace the prednisone. After 5 years he puts his patients back on 5ASAs. Protocol may have changed since the late 1990s hence your doctor's advice.
dx proctitis in 1987
dx UC in 1991, was stable until 1998
1998 started prednisone, asacol, pentasa, nortriptylene, ativan, 6MP, rowasa enemas and suppositories, hydrocortisone enemas, tried the SCD diet, being a vegetarian, omega 3s, flax, pranic healing, yoga, acupuncture, probiotics
2000 lost all my B-12 stores and became anemic
2001 opted for j-pouch surgery- now living life med-free