I too had excoriating IBS while on Tamoxifen and could find nothing about it on the web or from the nurses or doctors at the Dana Farber here in Boston, which is considered one of the finest cancer hospitals in the world. Go figure. Here is my experience and what I learned from it.
I have always suffered from IBS, but three months on Tamoxifen pushed it through the roof! What had been a chronic, manageable issue had become acute. I hit the web and found a promising description of Bacterial Overgrowth of the small gut. Stopped eating carbs – my pain stopped by the end of first day.
After more research I discovered that Tamoxifen floods your gut with estrogen, just like when you are pregnant. Estrogen in your gut slows your digestion down so you can absorb more nutrician for your baby. And since normally your small gut is ALWAYS moving to push the bacteria in your food through quickly, making your small gut too slow can lead to the bad bacteria multiplying. Most of those bad bacteria eat carbs and create huge gas which leads to terrible IBS. So cutting off their carb supply means no more gas and pain.
I stopped the tamoxifen (I’d had stage 1, grade 1 lobular – clean margin lumpectomy, nothing in nodes, low risk). After a a diagnosis of bacterial overgrow and a year of no carbs ( I lost too much weight believe it or not) I was working my way though useless antibiotics because the big gun I needed (Xifaxan 550mg) was 3,000 for a full dosage and BC/BS wouldn’t cover it until I’d tried the cheaper antibiotics. Then I got a bag stomach flu which triggered a volvulus (full twisting of the gut) which is about 1,000x more painful than childbirth plus life threatening. Rushed to hospital in ambulance, gut untwisted on its own suddenly, many hours later, I was allowed to go home and meet a surgeon the next day. Had the surgery to correct a previously undiagnosed birth defect and finally got the Xifaxan to fix my bacterial overgrowth.
Turns out one of the main underlying cause of my chronic IBS was a partially mal rotated small bowel. This is a very common, undiagnosed birth defect. The much improved imaging we now have is making the diagnosis more common. A fully mal rotated small bowel is the birth defect that causes newborns to be rushed into surgery after projectile vomiting their first meal. My partially mal rotated small bowel created a narrowing in my gut which slowed my digestion down and lead to gas and cramping. The tamoxifen slowed it down even further, leading to the bacterial overgrowth and the volvulus.
And now my IBS is practically gone! I still have a cranky gut, which does cramp and spasm on occasion, but basically it’s better than it has ever been and what issues I do have are totally manageable.
So Tamoxifen could easily be triggering a serious increase in you IBS. Try getting off Tamoxifen, if you aren’t already, and no carbs for a day to see if bacterial overgrowth might be the culprit. Good luck!