Taking multivitamins before a colonoscopy can interfere with the preparation process by darkening stool, leaving residue, or thinning the blood which makes it harder for doctors to clearly examine the colon. Ingredients like iron, vitamin E, and certain herbs are common culprits that reduce the effectiveness of the procedure. https://doxlet.com/blogs/news/should-you-take-a-multivitamin-on-the-carnivore-diet