MDR Rule 21: Why a Substance-Based Device Is Never Class I

EU MDR Annex VIII, Chapter III added four special rules the old directive never had, and Rule 21 is the one that catches products regulatory teams have already decided are "clearly low-risk" — a nasal spray, a wound gel, an absorbable surgical material. Rule 21 doesn't classify by what the product treats or how invasive it is. It classifies by where the substance goes and whether the body absorbs it. Get that second question wrong, and a device a team assumed was Class IIa can land two tiers higher.

What Rule 21 actually covers

Rule 21 applies to devices composed of substances or combinations of substances that are intended to be introduced into the human body via a body orifice, or applied to the skin, and that are absorbed by or locally dispersed in the human body. That scope is broader than the products a device team usually flags for special handling. Nasal decongestant sprays, absorbable surgical hemostats, keratolytic wart-removal gels, and barrier or wound-protection creams can all fall inside it — not because they look exotic, but because they are, chemically, a substance doing something to or in the body rather than a mechanical device acting on it.

The route-and-absorption test

Rule 21's text does not branch on device type, contact duration, or invasiveness the way most of Annex VIII does. It branches on two questions: where does the substance go, and does the body absorb it systemically to achieve the intended purpose? Those two questions, worked through in order, are what actually sets the class — not an intuition about how mild the product's action looks.

• Class III — systemic absorption to achieve the intended purpose. The device, or its metabolism products, are systemically absorbed by the body in order to work — wherever in the body that absorption happens.
• Class III — the stomach/lower-GI carve-in. A separate trigger: if the device achieves its intended purpose in the stomach or lower gastrointestinal tract and the substance or its metabolites are systemically absorbed, it is Class III even if the team's first read was "local GI action."
• Class IIb — all other cases. The true catch-all. Any route-and-absorption profile that clears both Class III triggers and doesn't fit the narrow Class IIa carve-out below defaults here.
• Class IIa — skin, or nasal/oral cavity as far as the pharynx, with no systemic absorption. The device is applied to the skin, or introduced via the nasal or oral cavity only as far as the pharynx, and achieves its purpose there without the substance or its metabolites reaching systemic circulation.

The pharynx is not a figure of speech in Rule 21 — it is the literal line the rule draws. A spray that acts in the nasal cavity is a different classification question from one whose droplets are swallowed and absorbed past it. Why the site-of-action boundary carries the weight

Not the borderline question

Rule 21 gets conflated with a different, earlier question: whether a substance-based product is a medical device at all, rather than a medicinal product. That is the separate device-versus-medicinal-product question MDCG 2022-5 and Article 117 govern, decided by the product's principal intended mode of action. Rule 21 does not touch that determination — it starts from the assumption that the borderline question is already settled in the device's favor, and then decides which class of device the product is. A team that runs the Rule 21 route-and-absorption test on a product whose device status is still unresolved is answering the wrong question first.

1. Confirm device status first. Settle the borderline determination before spending time on Rule 21's absorption test — a product that is actually a medicinal product doesn't reach Rule 21 at all.
2. Map the exact route and site of action. Skin, nasal/oral cavity to the pharynx, or introduced further — esophagus, stomach, lower GI tract, or another orifice — each points toward a different outcome.
3. Settle the absorption question last. Local action only, or systemic absorption of the substance or its metabolites, including the stomach/lower-GI carve-in that applies on its own terms.

Where this quietly bites

The failure mode is rarely a team ignoring Rule 21 outright. It's a team that classifies the product once, early, on an intuitive read — "it's just a nasal spray, that's low risk" — and never revisits the absorption question when a formulation changes. A reformulated nasal product that now clears the pharynx and is swallowed, a wound-care substance whose excipient change increases dermal absorption, or a GI-acting device whose mechanism turns out to involve systemic uptake of a breakdown product can each move a product from an assumed Class IIa into Class IIb or III without anyone deciding to reclassify it. As with Rule 19's nanomaterial test, the risk isn't that the rule is obscure — it's that a formulation or supply-chain change never gets flagged back to the classification file that depends on it.

None of this is exotic once the two questions are separated: is it a device, and if so, where does it go and does the body absorb it. What makes Rule 21 dangerous is treating a substance-based product's classification as a one-time judgment call rather than a route-and-absorption argument that has to survive a Notified Body reading it cold — and re-running it whenever the formulation moves. If your last classification review predates a formulation or excipient change, that is where to start; our EU MDR device classification work exists for exactly this gap.