Can Gastrografin (contrast medium) cause black tarry diarrhoea?

My mother aged 82 who has had a previous CA sigmoid colon, has just been for a CT check up and today the day after scan and the contrast medium, she has tarry loose stools. I know it can cause diarrhoea, but was shocked to find out it was black. This is what she had before surgery 15 months ago. She also had a slight rise in her Cancer marker blood tests to 8.4. She had a reversal ileostomy two weeks after initial surgery.

Black tarry stool is usually a sign of GI bleeding. She should be watched closely for signs of shock / blood loss (elevated heart rate, pale skin, rapid breathing, mental status changes, anxiety, thirst) and should be evaluated by a physician immediately. Get her to the ER or urgent care.

I agree with the poster above. Go to the ER or the urgent care. I think if black tarry stools was to be expected after the CT they would have mentioned it when they explained the test to you...

You are right, this is unlikely to be a side effect of the gastrografin. Black tarry stool is usually melena=changed blood. This may be quite serious and you should seek medical advice quickly!