Despite the early promise of a few limited studies, there is no consensus as to the optimum oral contrast type, dose and concentration. Anecdotally barium tends to best label solid residue whereas iodinated contrast best labels fluid, but this assumption has not been proven. Furthermore it is known that iodinated contrast tends to draw fluid into the bowel, producing a wet colon, which may not be suited to CT colonography. However, this indrawing of fluid by iodinated contrast such as gastrograffin produces a mild laxative effect, which may obviate the need for further formal purgation.