Iron oxide with E-number 172 are yellow, red, brown and black dyes made from oxidised iron. Capsules are added, for example, to protect the active substances against degradation from sunlight.
Iron oxide used as an excipient is chemically pure and practically insoluble in water. The amount of iron is already small and is only absorbed in such an insignificant amount by the body. Iron oxide used as an excipient can be used by people who have opted out of iron supplements. The natural variations of iron from the diet far exceed the content of iron oxide as an excipient.
Experiments have shown that a maximum of 0.054% dissolves under conditions corresponding to that in the gastrointestinal tract. The amount absorbed from dietary supplements is physiologically completely irrelevant as an iron source.
Since both iron and oxygen are elements, the raw material can be called natural. Iron oxide (Fe3O4) is iron that is oxidized, that is, bound to oxygen. Naturally produced iron oxide in some forms of approved supplements do not exist. Iron that rusts in nature is admittedly natural, but chemistry would still have to be used to clean it.
Iron oxide is only unhealthy if inhaled.