<p>Iohexol provides opacification of blood vessels and permits radiographic visualisation until sufficient haemodilution occurs or sufficient contrast medium has left the site of injection. Being a non-ionic compound, Iohexol yields solutions of lower osmolality than the conventional ionic contrast media. Intravenous or intra-arterial injection of Iohexol causes less pain and sensation of heat than conventional ionic media with similar iodine content. Iohexol solutions cause less cardiac and vascular disturbances on intravascular injection. The transit time of Iohexol through the coronary vascular system is slightly increased compared with conventional ionic contrast media, probably due to the increased viscosity of Iohexol at comparable iodine concentrations. The period of maximal opacification of the renal vessels may begin as early as 30 seconds after IV injection. Urograms become apparent in about 1 to 3 minutes with optimal contrast occurring between 5 to 15 minutes. In nephropathic conditions, particularly when excretory capacity has been altered, the rate of excretion may vary unpredictably, and opacification may be delayed after injection. Severe renal impairment may result in a lack of diagnostic opacification of the collecting system. The initial concentration and volume of the medium, in conjunction with appropriate patient manipulation and the volume of CSF into which the medium is placed, will determine the extent of diagnostic contrast that can be achieved. Following subarachnoid injection, Iohexol will continue to provide good diagnostic contrast by conventional radiography for at least 30 minutes. Slow diffusion of Iohexol takes place throughout the CSF as well as transfer into the circulation. At approximately 1 hour, contrast of diagnostic quality will not usually be available for conventional myelography. However, sufficient contrast for CT myelography will be available for several hours. If CT myelography is to follow, it should be deferred for several hours to allow the degree of contrast to decrease. Following lumbar subarachnoid placement, irrespective of the position in which the patient is later maintained, slow upward diffusion of Iohexol takes place throughout the CSF. CSF contrast enhancement for CT scanning may be expected in the thoracic region in about 1 hour, in the cervical region in about 2 hours and in the basal cisterns in 3 to 4 hours after administration into the lumbar subarachnoid space.</p>