A formulation team evaluating a molecule for pediatric use often starts from the adult formulation and works backward: adjust the dose, adjust the volume, carry the excipients over. The earliest signal that this sequence will run into trouble is usually already sitting in the excipient limits, well before the formulation reaches a pediatric patient.
Propylene glycol’s tolerance limit for adults doesn’t carry over to newborns. The neonatal threshold is a small fraction of the adult one, and benzyl alcohol is contraindicated outright under age three regardless of dose. That gap isn’t rare: one study of EMA- and FDA-authorized pediatric oral medicines found that while more than 80% had an approved oral formulation, roughly half weren’t age-appropriate for the children taking them.
Both limits trace back to the same mechanism: a neonate’s liver and kidneys clear these compounds far more slowly than an adult’s, so a proportionally smaller dose can still accumulate faster than the body handles it. Reading that signal at the candidate stage, rather than after a pediatric study surfaces it, is what separates a formulation strategy from a formulation problem.
Which Excipients Carry the Tightest Pediatric Limits?
Propylene Glycol
Propylene glycol solubilizes poorly water-soluble APIs in liquid formulations, and adult metabolism clears it efficiently enough to support a comparatively high tolerance threshold. EMA guidance sets the pediatric threshold far lower, closer to 50 mg/kg/day under age five and near 1 mg/kg/day in term neonates.
The clearest documented case is lopinavir/ritonavir oral solution, which contains both ethanol and propylene glycol at concentrations that led to cardiac, renal, and respiratory complications in premature neonates, prompting an FDA label restriction in 2011.
Benzyl Alcohol
Benzyl alcohol preserves multi-dose liquids against microbial growth, and adult patients metabolize it without incident. Infants, particularly those born at low birth weight, lack that metabolic capacity, and the compound can accumulate to dangerous levels rather than clear normally, which underpins the age-three contraindication regardless of dose.
Ethanol
Ethanol serves as a co-solvent across a wide range of adult liquid formulations, and its pediatric tolerance sits well below the adult threshold on its own. The bigger exposure risk comes from combination: a child taking more than one ethanol-containing product accumulates exposure across every dose taken that day, not just the one under review.
Six Things That Influence the Drug Delivery Format
1. Solvent System
Liquids need a way to get the API into solution, and that’s typically where propylene glycol and ethanol come in. Oral thin film and buccal film formulations avoid that step almost entirely, generally built around cellulose derivatives like hydroxypropyl methylcellulose (HPMC) or the natural polymer pullulan, plasticized with glycerol instead of dissolved in a solvent.
For a pediatric program, that difference removes the excipient class carrying the tightest age-based limits from the formulation entirely, rather than trying to dose it within a lower ceiling.
2. Taste Masking
In pediatric patients, palatability is the difference between a dose taken and a dose refused. A bitter API in a liquid typically gets masked with sweeteners and flavoring agents added at meaningful concentrations, stacking more excipient load onto an already constrained formulation.
Film can mask taste at the particle level instead, often by coating the API in a barrier like ethyl cellulose, or through microencapsulation or ion-exchange resin techniques, before it’s suspended in the film matrix.
3. Dose Flexibility
Pediatric dosing spans a wide weight range within a single indication, often by a factor of ten or more between the smallest and largest patient. Conventional tablets are manufactured at a small number of fixed strengths, and adding a new one means new compression tooling and a new bulk manufacturing batch.
A film’s unit dose can be adjusted by changing thickness or surface area within the same casting and drying process, without the tooling change a new tablet strength requires. Each new strength still needs its own SKU and, absent an FDA-agreed bracketing approach, its own stability data, but the underlying manufacturing platform doesn’t change the way it would for tablets.
4. Administration Requirement
Swallowing coordination develops gradually and isn’t reliable in every child, particularly the youngest patients and those with certain neurological or developmental conditions. A conventional tablet or capsule requires coordinated swallowing to move the dose past the pharynx intact, along with enough liquid to clear it. Film formats dissolve or adhere directly against oral mucosa and require neither.
5. Route Bypass
A transdermal patch skips the oral excipient question altogether: the patch contains no solvent, preservative, or liquid vehicle while the patient wears it. Solvent is still used earlier, during the blending step that prepares the drug-adhesive intermediate before it’s coated and dried onto the backing, but that’s a manufacturing-process consideration, not a patient-exposure one. The conversation shifts instead to adhesive systems, acrylic or silicone-based, and permeation enhancers.
For pediatric patients, that shift also removes the cooperation an oral dose demands, which matters most in chronic conditions where a caregiver is administering the dose daily.
6. Regulatory Precedent
Sponsors are increasingly expected to plan for pediatric use during development, under FDA guidance issued specifically to close that gap, rather than discovering these same limits once a required pediatric study forces the issue. A format with no pediatric approval history makes for a harder regulatory case, no matter how sound the underlying science.
The methylphenidate transdermal system, approved for ADHD in patients ages 6 to 17, gives a chronic pediatric transdermal program a working template already reviewed once.
A sponsor citing an approved pediatric transdermal system is working from a pathway FDA has already evaluated for this population, rather than asking a review division to weigh the format and the age group for the first time simultaneously.
READ MORE: Transdermal Patches 101: What They Are, How They Work & Why Patients Like Them
Conclusion
A pediatric patient’s biology doesn’t scale down from an adult’s. Neither should the formulation.
That’s the real throughline connecting the excipient limits above to the six factors that follow them. A solvent system, a taste-masking strategy, a dosing range, an administration route: none of these are separate decisions made in sequence. They’re one decision, made at the candidate stage, about whether the format was built for the patient in front of it.
Making that decision at the candidate stage is the evaluation ARx runs across oral thin film, buccal film, and transdermal platforms, from early feasibility through commercial manufacturing, as both a drug delivery partner and a CDMO.
Working through a pediatric formulation strategy? Connect with the ARx team to discuss your molecule and the excipient constraints specific to your target age range.