Omnipaque is an iodine based radiopaque contrast dye used in the diagnosis of vascular conditions (like pulmonary embolism). It is usually injected into the spinal fluid or the blood stream. It has a moderate/large molecular weight at 821 Dalton and is very poorly bound to protein. It is a pregnancy class B drug (which means it is more safe to the unborn child than most drugs). It is "very poorly absorbed from the gastrointestinal tract". It is rapidly excreted through the urinary system with 99% being excreted unchanged in 24 hours. Here is why I think this drug is okay.
1) The molecular weight of 821 means that it is unlikely to pass into the breast milk since it is a large molecule and may be too large to enter the milk in the first place. Molecular weight isn't the only thing that dictates passage, but unless it is bound to a protein and carried actively into the lactocyte it is unlikely to enter the milk (its just too big to get in easily).
2) It is unbound to protein, now that generally means that there is free drug in the blood stream and thus it is more easy to leak into the milk. if that was all I knew about this drug, I would worry. But it isn't.
3) The oral bioavailability of this drug is only 0.1-0.5% that means that almost nothing is absorbed through the stomach. Now here is an EARTH SHATTERING idea, Breast milk is an oral drug. That means that anything that makes it into the breast milk has to be absorbed through the gut to enter and harm the baby. In the case of this drug, almost nothing is absorbed in the gut. IF any omnipaque entered the breast milk it would simply pass through the stool undigested, un-absorbed, and without harm. When I first heard Dr. Thomas Hale say that in a conference it literally blew me away! I had never thought of that before. So as a general rule, we give people IV drugs because they can't pass through the gut very well (Morphine, Gentamycin, IV Contrast dye). So an IV only drug is almost always perfectly safe. Even if 100% of the drug entered the breast milk less than 1/2% of that dose would enter the baby to be excreted by the kidneys. That is nothing.
4) A class B drug. That means that there is no evidence of harm to the developing baby. If you can give this drug to a pregnant mother without risk (and those kidneys are VERY premature) then I am confident that it won't hurt the newborn. The only exceptions to that rule are drugs that are sedating (narcotics and such) the unborn baby isn't breathing, so respiratory depression or sedation wouldn't harm him but it can harm a newborn who is responsible for breathing on his own.
5) It is rapidly excreted in the kidneys, that means that if anything entered the blood stream, what little entered would be rapidly cleared. It wouldn't accumulate in the blood, the fat, or any of the baby's organs.
6) It is safe to use the drug on children. There is no restriction of this drug for pediatric use. The FDA flier doesn't specifically state it is safe to use in newborns, but it clearly shows that it is safe for pediatric use. If there was a restriction the FDA would report that, and they don't. If you can inject it into a newborn without risk of injury, you can probably allow it to go into his mouth without injury.
So yes, while I accept that it is poorly studied, I can't see any reasonable reason to worry about this drug harming the newborn.
Tom
References: Hale and Hartman (2007) Textbook of Human Lactation (chapter 24 "The transfer of medications into human milk).
FDA (2016) Medication information - Omnipaque. Retrieved from: http://www.accessdata.fda.gov/drugsatfda_docs/label/2015/018956s095,020608s031lbl.pdf on 2 Feb 2017.
