It all depends. Most spices, such as turmeric, ginger, peppermint, cumin, coriander are soothing to the sensitive gut, so there is no need to avoid spicy food if you have IBS. Even pepper is not a problem for the sensitive gut. The only spice that can make the symptoms of IBS worse is chilli and even that depends on how frequently it is taken.
Acute ingestion of chilli may causes burning in the mouth and the abdomen, abdominal pain and after a few hours increased sensitivity in the rectum in the rectum too. This effect is greater in patients with IBS-diarrhoea. However we all know that our tolerance to chilli increases if we ingest it regularly. People who consume chilli regularly hardly ever notice how hot it is.
A recent study from Thailand on 16 people with IBS-diarrhoea has shown that although there is an increased sensitivity to begin with, daily ingestion of a capsule of chilli taken before a spicy meal chilli for 6 weeks reduced abdominal burning in response to a spicy meal and also reduced rectal sensitivity. Chilli stimulates the specific pain receptor, ‘transient receptor potential vanilloid 1’ (TRPV1), but this after a period of accommodation this effect is reversed and sensitivity is reduced. This has led some to think that longer term ingestion of chilli might actually be a treatment for the sensitive gut, but this study does not back that up. This might be due to a lack of power due to the low number of participants, the relatively low dose of chilli – about half of the normal daily consumption in Thailand and the fact that the presence of abdominal burning meant that subjects knew what treatment they were getting.
It may not be the chilli that stimulates symptoms in a sensitive gut. Chilli is often served with curries, tacos, chilli dogs, and other meals that are not only rich in fat, but also contain onions, garlic, beans and other high FODMAP foods. So it is possible that it is the fat or the FODMAPs that is generating the symptoms and nothing to do with the chilli.