Nonionic contrast agents are a group of contrast media that do not dissociate into charged particles. Many of the side effects of contrast agents are due to the injection of a solution with high osmolality compared to blood. Nonionic contrast agents have a low osmolality and tend to have less toxicity. However, as with all iodinated contrast agents, serious or fatal reactions have been associated with their use.
Due to their lower neurotoxicity, nonionic contrast agents can also be used for myelography. The only FDA approved agent, iodixanol, is iso-osmolar to blood, which makes it preferable for examinations of the central nervous system and cardiovascular studies. Because of its higher costs relative to other nonionic agents, the use is sometimes reserved for patients with high risk of nephrotoxicity.

See also Nonionic Dimer, Nonionic Monomer and CM Ratio.

(LOCM) Low-osmolar contrast media have a wide range of indications due to their lower side effects. The type of contrast media is an important risk factor for an adverse reaction.
LOCM have not completely replaced contrast media with higher osmolality due to their higher cost. Guidelines of professional organizations give recommendations for the selective use of low-osmolar contrast media for certain high-risk patients.
There are ionic and nonionic iodinated contrast materials with low osmolality available:
An adverse reaction occurs in low-risk patients who receive conventional ionic contrast agents more often than in high-risk patients who receive nonionic LOCM.

See also Contrast Enhancement, Biliary Contrast Agents, Safety of Contrast Agents and Contrast-Induced Nephropathy.

Nonionic monomers are used as x-ray contrast agents. To create a nonionic monomer, the tri-iodinated benzene ring is made water soluble by the addition of hydrophilic hydroxyl groups to organic side chains (CM ratio=3). Nonionic monomers have an intermediate osmolarity, intermediate viscosity and elevated hydrophilicity with three atoms of iodine per molecule. Lacking a carboxyl group, nonionic monomers do not ionize in solution. A nonionic monomer is potentially less chemotoxic than an ionic monomer.
Common nonionic monomers are iohexol (Omnipaque), iopamidol (Isovue®), ioversol (Optiray®), and iopromide (Ultravist®). Nonionic monomers are contrast agents with a wide range of indications due to their nonionic nature and lower osmolalities.

A nonionic dimer consists of 2 joined nonionic monomers. Nonionic dimers are used as x-ray and CT contrast media.
Nonionic dimers have the lowest osmolarity. These contrast materials contain 6 iodine atoms for every 1 particle in solution (CM ratio=6). Modern CT techniques require fast bolus injections and thus low-viscosity, high-concentration, well-tolerated contrast agents. Nonionic dimers have the highest viscosity, which limits the clinical usefulness. Examples of nonionic dimers are iotrolan (Isovist®) and iodixanol (Visipaque).

See also Contrast Agents.