Depends on the country. In Sweden it's illegal to have certain drugs in your system. E.g. if THC is found in your blood you can be convicted for a minor drug offense. We're sadly pretty extreme with regards to drugs though.
E.g., with alcohol, the “in the blood is illegal” rule would criminalize auto-brewery syndrome occuring in those legally denied alcohol (e.g., due to age), while the “in the blood is admissible as evidence of use” would not, though those with the syndrome might need to present evidence of it to avoid charges.