Dependent on extent, duration,age at diagnosis and severity of disease. I think much of the data is faulty because so many of the colon cancer cases in UCers were people that did not use the current drugs.
Here is a hack and paste from a NEJM article:
NEJM said...
Ninety-two cases of colorectal cancer occurred in 91 patients. As compared with the expected incidence, the incidence of colorectal cancer in the cohort was increased (standardized incidence ratio [ratio of observed to expected cases] = 5.7; 95 percent confidence interval, 4.6 to 7.0). Less extensive disease at diagnosis was associated with a lower risk; for patients with ulcerative proctitis, the standardized incidence ratio was 1.7 (95 percent confidence interval, 0.8 to 3.2); for those with left-sided colitis, 2.8 (95 percent confidence interval, 1.6 to 4.4); and for those with pancolitis (extensive colitis, or inflammation of the entire colon), 14.8 (95 percent confidence interval, 11.4 to 18.9). Age at diagnosis and the extent of disease at diagnosis were strong and independent risk factors for colorectal cancer. For each increase in age group at diagnosis (<15 years, 15 to 29 years, 30 to 39 years, 40 to 49 years, 50 to 59 years, and ≥60 years), the relative risk of colorectal cancer, adjusted for the extent of disease at diagnosis, decreased by about half (adjusted standardized incidence ratio = 0.51; 95 percent confidence interval, 0.46 to 0.56). The absolute risk of colorectal cancer 35 years after diagnosis was 30 percent for patients with pancolitis at diagnosis and 40 percent for those given this diagnosis at less than 15 years of age.
The American College of Gastroentology Guidelines mention that increased risk for cancer begins after 10 years for pancolitis but distal disease is 4 times that or 40 years.
You can see that UP overal only increases risk over a lifetime by 1.7 times.