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.
Is it illegal to have THC in your system or is having THC in your system admissible as rebuttable evidence of possession/use of illegal drugs? Either would support the outcome you describe, but they are meaningfully different.
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.
It's the latter: (intentional) use of (illegal) drugs is illegal, and having THC in your blood is proof enough to convict you (even if you've consumed it in a different country where it's actually legal; there's precedent). Typically you'd just get a fine though, if you admit guilt and don't go to court. See § 1 here for the exact law: http://www.notisum.se/rnp/sls/lag/19680064.htm
