The 'inactive' drug ingredients fueling health problems
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“Anytime I see a patient with multiple medication allergies, I start thinking about the additives. It’s what I call the ‘perfect storm.’” — Kara Wada, MD, assistant professor in the Department of Otolaryngology–Head and Neck Surgery at the Ohio State University College of Medicine
“While labeled 'inactive,' many excipients can trigger clinically significant reactions, particularly in neurologically sensitive patients." — Rizwan Bashir, MD, a board-certified neurologist at AICA Orthopedics
In the US, more than 90% of oral medications contain at least one “inactive” ingredient known to cause allergic reactions or intolerances in certain patients.
Despite being classified as inert, many of these excipients, including preservatives, dyes, and stabilizers, have been classified as adverse reaction–associated inactive ingredients (ARAII).[][]
Now, a growing number of experts say it’s time for prescribers to treat these additives with the same clinical scrutiny as active ingredients.
The ingredients most doctors overlook
These ingredients are commonly used to improve stability, absorption, or manufacturing ease. But they can also lead to hypersensitivity reactions, especially in patients with pre-existing allergies, autoimmune conditions, or gut dysbiosis.
“In my world, where I’m looking at everything through the lens of allergy, immunology, and functional medicine, I see the fallout from this [excipients] on a regular basis,” says Kara Wada, MD, assistant professor in the Department of Otolaryngology–Head and Neck Surgery at the Ohio State University College of Medicine. “Anytime I see a patient with multiple medication allergies, I start thinking about the additives. It’s what I call the ‘perfect storm.’"
It’s rarely one single cause, Dr. Wada says. More often, an excipient is the final straw for an already stressed immune system, especially if a patient's gut microbiome is already out of balance.
Related: This 'probably toxic' ingredient is in 4 of the most-prescribed drugs—and the FDA may soon outlaw itDyes
While labeled "inactive," many excipients can trigger clinically significant reactions, particularly in neurologically sensitive patients, according to Rizwan Bashir, MD, a board-certified neurologist at AICA Orthopedics.
“I’ve seen adverse responses associated with dyes, like FD&C Red No. 40, and preservatives, such as sodium benzoate—especially in pediatric populations with seizure disorders or ADHD," Dr. Bashir says. "These ingredients can exacerbate neurobehavioral symptoms or cause allergic-like reactions that complicate diagnosis. Patients with polypharmacy or known sensitivities are especially vulnerable, yet current labeling often lacks the granularity needed for proactive screening.”
PEGs, polysorbates, and propylene glycol
PEG has received more attention in recent years due to its association with allergic reactions to COVID-19 mRNA vaccines.[]
But PEGs and their chemical cousins, polysorbates and propylene glycol, are common across drug classes—from injectables to oral tablets to topical creams.
“Beyond the usual suspects, like lactose, gluten, and dyes, we have to keep a close eye on propylene glycol and polyethylene glycol,” says Dr. Wada. “These are everywhere—not just in prescriptions, but in cosmetics and a ton of ultra-processed foods. You have to think about the total load. When a patient is on multiple meds, the cumulative effect of these so-called 'harmless' additives can be significant.”
Underrecognized preservatives
Benzalkonium chloride (BAC), a quaternary ammonium preservative used in ophthalmic and inhaled medications, has been shown to impair mucociliary clearance and worsen asthma symptoms.[] Yet it remains common in nasal and eye drops.[]
Another additive, carboxymethylcellulose (CMC), has been documented in multiple case reports to cause anaphylaxis following joint injections or exposure via nasal sprays.[]
Related: 5 drug-food interactions to bewareWhat should physicians do?
Currently, there are no FDA-approved panels to screen broadly for excipient sensitivity. Skin testing is limited to a few agents (PEG, polysorbates, PVP, gelatin, alpha-gal), and even then, sensitivity varies widely.
According to Dr. Wada, these risks are easy to miss in a standard clinical setting. “Unfortunately, there’s no perfect test,” Dr. Wada says.
"We often dig into the full ingredient list, not just the active medications. It’s a level of detail the standard 15-minute appointment just doesn't allow for, which is a huge flaw in the system," Dr. Wada says. “My most powerful tool is simply listening to the patient's story and believing them. If the history points to a classic, immediate allergy, skin testing can give us answers. But for most sensitivities and intolerances, the gold standard is still a carefully managed elimination and re-challenge. It’s highly personalized; there’s no one-size-fits-all protocol when you’re dealing with this level of immune complexity.”
The experts that MDLinx interviewed recommend doing the following to avoid excipient-induced adverse events in patients:
Review full ingredient lists, including excipients, for every medication change.
Look for cumulative exposure across medications, supplements, and personal care products.
Consider elimination-rechallenge when testing isn’t available or conclusive.
Report suspected excipient reactions to FDA’s MedWatch database. []