Molecular weight, grade, and application… Old Pharma’s advice for an informed choice of the most appropriate PEG.
Polyethylene Glycols (PEG) represent an extremely versatile family of polymers used in numerous pharmaceutical applications.
PEG 200, PEG 400, PEG 1500, or PEG 4000 belong to the same chemical family, but they feature different average molecular weights and physicochemical characteristics, which influence their possible applications.
Scientific literature emphasizes how the selection of the appropriate PEG must take into account its physicochemical properties and the required function in the formulation. Indeed, PEGs find application in multiple pharmaceutical systems, from solubilization and plasticization to more complex drug delivery technologies.
The question to ask, therefore, is not simply “which PEG?”, but which PEG, with which grade, for which application?
PEG as an API or excipient?
Within the polyethylene glycol family, some PEGs are used primarily as excipients, while others can also play a role as Active Pharmaceutical Ingredients (APIs).
A useful classification can be:
- Primarily pharmaceutical excipients: PEG 200, PEG 300, PEG 400, PEG 600, PEG 1000, PEG 1500, PEG 6000, PEG 8000
- APIs and, depending on the grade and application, also excipients: MACROGOL 3350, MACROGOL 4000
However, molecular weight alone does not determine suitability for a particular use. Intended use, quality grade, specifications, impurity profile, pharmacopoeial requirements, and regulatory package must be evaluated together.
PEG as excipients: molecular weight influences function
Lower molecular weight PEGs
PEGs such as 200, 300, 400, and 600 represent important pharmaceutical excipients.
Depending on the specific PEG and grade, they can be used in liquid and semi-solid formulations with functions such as solvent, vehicle, solubilizer, or other functional component of the formulation.
Even within the same molecular weight, there may be grades intended for different applications. Specifications, purity, impurity profile, intended route of administration, and documentation associated with the product therefore become essential elements in the selection process.
Intermediate and high molecular weight PEGs
As molecular weight increases, the physical state and technological properties of the material progressively change.
PEGs such as 1000, 1500, 3350, 4000, 6000, and 8000 can therefore meet different formulation needs and find application in various pharmaceutical systems. The selection of the specific grade must be correlated to the characteristics required by the final product.
It is therefore more useful to consider PEGs as a formulation platform, in which different molecular weights and grades allow for meeting diverse needs.
When PEG becomes an API: Macrogol 3350 and Macrogol 4000
A particularly interesting feature of the PEG family concerns Macrogol 3350 and Macrogol 4000, which, in addition to possible applications as excipients, are also established as active pharmaceutical ingredients.
A 2025 scientific review describes macrogol used in the treatment of constipation as a PEG in the range of approximately 3,500–4,000 Daltons. Through interaction with water and with negligible gastrointestinal absorption, macrogol helps increase water content and luminal volume, promoting evacuation.
Macrogol 3350 as an API
The Macrogol 3350 is an established API in the treatment of constipation and can also be found in formulations associated with electrolytes.
In a multicenter, randomized, placebo-controlled study in adults with chronic idiopathic constipation, PEG 3350 at the studied dose of 17 g/day reached the response endpoint in 42% of patients compared to 13% with placebo, with sustained efficacy over the 24 weeks considered.
PEG 3350 + electrolyte formulations also have clinical evidence, including in the pediatric population.
Macrogol 3350 therefore represents an ingredient of particular interest for pharmaceutical companies active in the gastrointestinal sector and for manufacturers of laxative formulations, including specific formulations with electrolytes.
Macrogol 4000 as an API
The Macrogol 4000 is also widely used as an API in the treatment of constipation.
A review dedicated to Macrogol 4000 without electrolytes documents its use in chronic idiopathic or functional constipation in adults and children.
Macrogol 4000 has an established presence in oral formulations, including those without electrolytes, and is therefore of particular interest to pharmaceutical companies, consumer healthcare, and manufacturers of oral powders and sachets.
Which PEG is best suited for your project?
API or excipient? Macrogol 3350 or Macrogol 4000?
The Old Pharma team can support you in evaluating technical, qualitative, and regulatory needs and in identifying the most appropriate PEG for your application.
Contact us to discuss your project and receive more information on available PEGs.
Scientific references
D’Souza AA, Shegokar R. Polyethylene glycol (PEG): a versatile polymer for pharmaceutical applications. Expert Opin Drug Deliv. 2016;13(9):1257-1275. DOI: 10.1080/17425247.2016.1182485. PMID: 27116988. [pubmed.ncbi.nlm.nih.gov]
Menees SB, Lembo AJ, Chey WD. Polyethylene Glycol 3350 in the Treatment of Chronic Idiopathic Constipation: Post hoc Analysis Using FDA Endpoints. Can J Gastroenterol Hepatol. 2022;2022:3533504. PMID: 36120087. [pubmed.ncbi.nlm.nih.gov]
Bekkali NLH et al. Polyethylene Glycol 3350 With Electrolytes Versus Polyethylene Glycol 4000 for Constipation: A Randomized, Controlled Trial. J Pediatr Gastroenterol Nutr. 2018;66(1):10-15. PMID: 28906317. [onlinelibrary.wiley.com], [europepmc.org]
