Natamycin is not harmful at the levels found in food. Your body doesn’t absorb it. When you eat natamycin on cheese or other treated foods, it passes through your digestive tract essentially intact and is eliminated in your stool. The amounts used in food are also tiny, capped at 20 parts per million in finished products in the United States.
That said, natamycin has a more nuanced story than “completely harmless.” Here’s what you should actually know.
What Natamycin Is and Why It’s in Your Food
Natamycin is a naturally produced antifungal compound made by a soil bacterium called Streptomyces natalensis. It works by binding to a specific component of fungal cell membranes, which is why it kills mold and yeast effectively while having very low toxicity to human cells (our cell membranes don’t contain that target molecule). On food labels, you might also see it listed as pimaricin or, in Europe, E235.
Its primary job in food is simple: keeping mold off cheese. The FDA approved natamycin as an antimycotic (antifungal) agent for cheese in 2001, and it’s now used on shredded cheese, sliced cheese, and some other dairy and meat products. It’s also used medically as an antifungal eye drop for certain eye infections, which gives us additional human safety data.
Why Your Body Barely Notices It
The strongest safety argument for natamycin is that it essentially doesn’t enter your bloodstream. In animal studies, roughly 90% of an oral dose passed through the gut and came out in feces unchanged. Only about 5 to 7% showed any sign of absorption in rats, and dogs excreted most of it within 24 hours.
In humans, the picture is even clearer. When volunteers took 500 mg of natamycin (a dose thousands of times higher than what you’d get from eating cheese), researchers could not detect any natamycin in their blood at all, with a detection limit of 1 microgram per milliliter. A substance that doesn’t reach your bloodstream has very limited ability to affect your organs or tissues.
What Happens at High Doses
While the trace amounts on food are a non-issue, researchers have tested much larger doses to understand where problems begin. At oral doses of 300 to 400 mg daily (again, vastly more than food exposure), some people experienced nausea, vomiting, and diarrhea. In a group of 10 patients taking natamycin for fungal infections at doses ranging from 50 to 1,000 mg per day over periods of 13 to 180 days, digestive symptoms appeared in those receiving 600 mg or more daily. No blood cell changes were observed even at these high therapeutic doses.
To put this in perspective, the FDA limits natamycin in cheese to 20 mg per kilogram. You would need to eat an absurd quantity of treated cheese in a single sitting to approach even the lowest dose that caused mild side effects.
The Official Safety Limits
The Joint FAO/WHO Expert Committee on Food Additives (JECFA) has set an acceptable daily intake (ADI) for natamycin at 0 to 0.3 mg per kilogram of body weight. For a 70 kg (154 lb) person, that works out to 21 mg per day as the upper safe limit. This threshold was first established in 1976 and was reconfirmed as recently as 2024, unchanged.
Health Canada completed a re-evaluation of natamycin in 2024 and concluded that all current uses “meet current standards for protection of human health and the environment.” The only changes required were minor label updates for clarity.
Allergic Reactions Are Rare to Nonexistent
Among 111 patients treated with natamycin for various conditions, no allergic sensitization was reported. A separate survey of 73 workers who manufactured natamycin for an average of five years found no history of allergic reactions either. This is notable because occupational exposure during manufacturing would typically be much higher than what a consumer encounters.
The Resistance Concern Worth Knowing About
The one legitimate scientific concern about natamycin in food isn’t about direct toxicity to you. It’s about antifungal resistance. Natamycin belongs to the same family of antifungal drugs (polyenes) as amphotericin B, a critical medication used to treat life-threatening fungal infections in hospitalized patients.
Researchers have raised the question of whether low-level natamycin exposure from food could encourage resistant fungi to develop in your gut. Lab experiments have shown that polyene resistance can emerge when fungi are exposed to low, sub-lethal concentrations over many generations. When you eat natamycin-treated food, your intestinal yeast (like Candida species) is exposed to exactly that kind of low-dose environment.
So far, this concern remains theoretical. No one has demonstrated that eating natamycin-containing foods has actually produced resistant gut fungi in real-world conditions. But the concern is taken seriously enough that some researchers argue the use of any anti-infective agent as a food preservative should be kept to an absolute minimum to protect the usefulness of related drugs in clinical settings.
How Much You’re Actually Eating
If you eat shredded cheese, sliced deli cheese, or certain cured meats, you’re likely consuming small amounts of natamycin. The maximum allowed is 20 parts per million in the finished product, and much of it sits on the surface rather than penetrating deep into the food. Given that your body doesn’t absorb natamycin from the gut, and the amounts involved are a tiny fraction of the established safe daily limit, your actual systemic exposure is effectively zero.
If you’d prefer to avoid it, look for cheese sold in blocks you cut yourself (natamycin is typically applied to pre-cut or shredded products), or check ingredient labels for natamycin, pimaricin, or E235. Organic cheese standards in some countries restrict its use, though rules vary by region.