I took Keytruda five years ago for a metasis in my thigh (?) from an earlier lung cancer. After I took three infusions (is that about 10 weeks) I got colitis. Well the colitis could not be stopped as it is a known side effect which can happen to a very small percent of the population. Had to be hospitalized in hopes of controlling the 15 times a day diarrhea. Then picked up c diff in the hospital. The pain to my stomach - from the Keytruda - had me on around the clock morphine for about 4 weeks. Keytruda is a pretty serious drug and I recommend keeping yourself monitored about how and in which ways it can be affecting you. The doctors did find a drug to help the colitis - budinisone or such - and the pain in the stomach lifted and I went home very weak and thin. Couldn't eat the whole time so lost 30 pounds - had gotten ulcers on my duodeum. But I will say the cancer was eliminated. I was always fatigued from taking the Keytruda and accompanying chemo - could barely brush my teeth, shower, etc. Hard getting up from the toilet - if you can imagine. Very weak. The Keytruda web site says if Keytruda turns on you it is after the third infusion. About 10 weeks in. So keep an eye on yourself and definitely discuss with your oncologist. It's still a new drug - hope this helps!