Lisa K s not quite as far off base as some of you seem to think.
Ethylene Glycol is anti-freeze. Polyethylene glycol is a polymerized version, where the number such as 3350 represents the average molecular weight of the PEG molecules. The rub is that to make Polyethylene Glycol you need to start with Ethylene Glycol. The Polymerization process often leaves behinds small amounts of the un-polymerized product, and in fact in a chemical assay of Polyethylene Glycol, it is common to find very small amounts of the un-polymerized chemical.
This is why I am uncomfortable with a product that is 99% PEG, you don't know what the other 1% is unless an assay is provided with the product. The USP standard calls off maximum permissible amounts of impurities as well as what they are permitted to be. Any product that meets the USP standard or better (and is labeled Food Grade), will have at most very very small amounts of the un polymerized chemical. Ethylene Glycol is not like Cyanide where tiny amounts can kill you, It actually takes a pretty substantial dose to kill someone.
LD50 for Ethylene glycol is about 1.5 grams/Kilo, so the ingestion of even a few grams of Ethylene Glycol by a healthy 50 Kg adult is unlikely to be harmful. So as long as Ethylene Glycol impurity levels in the PEG product remains on the order of .1% of below, the is no hazard. However if you are buying PEG from chemical suppliers, beware that most will have a disclaimer that the product is not to be used for drugs, or human consumption. This is a warning that the impurities may in fact be toxic, (and may be a lot more toxic than ethylene glycol, i.e. heavy metals).
So my advice is stick to sources that your know provide a product that is safe for human consumption such as Miralax® and its competitors. As long as you do, the risk of ethylene glycol poisoning is nil.