text: What is Polyethylene glycol (PEG)?
  Polyethylene glycol (PEG) is a polyether compound derived from petroleum with many applications, PEG is the basis of a number of laxatives (as MiraLax). Whole bowel irrigation with polyethylene glycol and added electrolytes is used for bowel preparation before surgery or colonoscopy.
  PEG is also used as an excipient in many pharmaceutical products. PEG used in medicines for treating disimpaction and maintenance therapy for children with constipation.
  When attached to various protein medications, polyethylene glycol allows a slowed clearance of the carried protein from the blood.
  What is Propylene glycol (PG)?
  Propylene glycol is a viscous, colorless liquid, which is nearly odorless but possesses a faintly sweet taste. It is classed as a diol and miscible with a broad range of solvents, including water, acetone, and chloroform.
  It is produced on a large scale primarily for the production of polymers. In the European Union, it has E-number E1520 for food applications. For cosmetics and pharmacology, the number is E490. Propylene glycol is also present in propylene glycol alginate, which is known as E405. Propylene glycol is a compound which is GRAS (generally recognized as safe) by the US Food and Drug Administration under 21 CFR x184.1666, and is also approved by the FDA for certain uses as an indirect food additive.
  Polyethylene Vs Propylene Glycol
  Propylene glycol (PG) and polyethylene glycols or PEGs) are synthetic substances that are used as vehicles in various cosmetic and medicinal products.
  Propylene glycol is ubiquitously used in a variety of common items including edible items (sweeteners, whipped dairy products), cleaners, vaporizers, hand sanitizers and artificial tear preparations like Systane. It is a common cause of allergic contact dermatitis (ACD) and was named the ACD Society’s allergen of the year in 2018.
  In addition to causing contact dermatitis with cosmetic exposure, systemic contact dermatitis has been reported after oral ingestion. As PG is both a weak sensitizer and a cause of irritant dermatitis, patch testing interpretation can be difficult to interpret as testing results may not be robust if low concentrations are used and may lead to irritant response with higher concentrations. There are no reports upon literature review of IgE-mediated or anaphylactic reactions to PG.
  Polyethylene is also found in multiple cosmetic and industrial products and is used quite frequently in medicine as it is found in PEGylated medications, hydrogels, tablets and lubricants. Though thought to be fairly safe, but according to studies, there are multiple reports of immediate type hypersensitivity reactions consistent with anaphylaxis.
  A 2016 systematic review, noted 37 case reports of PEG induced reactions, 80% due to oral PEG exposure. Some cross reactivity does appear to exist between PEG and structurally related polymers such as polysorbate 80 and poloaxmer.
  PG and PEG have dissimilar chemical structures. PG is a small, single molecule with 3 carbons and 2 OH groups ( ie a double alcohol). PEG is a multi-unit polymer with a differing molecular weight and backbone. Historically the types of reactions these agents cause are dissimilar as noted above.
  Few studies have looked at sensitization to both agents. A single Turkish study retrospectively looked a patch testing for both PG and PEG as a marker for nitrofurazone allergy. Though not the main outcome of the study, a low proportion of subjects included (2 of 42) had positive patch testing to both PG and PEG. Thus there is not sufficient literature to support cross reactivity between PG and PEG. Though I am not sure which agent your patient has reacted to in the past, given how ubiquitous both agents are it would be prudent to take a detailed history as it is likely they are already tolerating exposure to the other agent which can provide some reassurance.”