Zero-tolerance limit: In everyday discussion, “zero-tolerance limit” describes the very low specified legal limits applied to commonly abused drugs in drug-driving law, including cannabis (THC).
Patients often hear the phrase even though the legal framework is more precise than that shorthand.
DfT says THC sits in the first group of specified drugs with low limits, while also explaining the statutory medical defence and the separate offence of driving while impaired.
It should not be used to imply there is no medical defence at all; there can be one if the medicine is prescribed and taken correctly.