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While the exact cause of IBD is not entirely understood, it is known to involve an interaction between genes, the immune system, and environmental factors.

Potential Causes of IBD

Immune system reaction

The body’s immune system usually eliminates foreign invaders (substances), such as bacteria, viruses, and fungi. Normally harmless bacteria (many of which aid in digestion) are protected in the GI tract. However, for people with IBD, the immune system reacts to these bacteria with inflammation. Environmental triggers initiate these immune responses, which can lead to chronic inflammation, ulceration, and thickening of the intestinal wall.


Scientific evidence does point to the role of heredity in IBD. In fact, studies have shown that 5% to 20% of affected individuals have a first-degree relative (parent, child, or sibling) with one of the diseases. While genetics is clearly a factor, the association is not simple. It is likely that more than one gene is at work, and just having the genes associated with IBD doesn’t absolutely predict that the disease will occur. These genes are known as susceptibility genes as they increase the chances for getting the disease. It is clear that other factors, including environmental factors, must also come into play.

Environmental factors

The environmental factors that trigger IBD are not known, but several potential risk factors have been studied, including:

  • Smoking — Active smokers are more than twice as likely as nonsmokers to develop Crohn’s disease. Surprisingly, the risk of developing ulcerative colitis is decreased in current smokers compared with people who have never smoked. The numerous potentially harmful health effects of smoking (e.g., cancer, heart disease) largely overcome any benefits of smoking for people with ulcerative colitis.
  • Antibiotics — The use of these medicines may increase the risk for IBD.
  • Nonsteroidal anti-inflammatory drugs (aspirin, ibuprofen, naproxen) — The use of these drugs may increase the risk for getting IBD and may worsen the condition.
  • Appendicitis in children — Children who undergo an appendectomy (removal of the appendix) are less likely to develop ulcerative colitis later in life. However, appendectomy in childhood may increase the risk for Crohn’s disease.