How long does Marijuana stay in the blood?

Q: Can you help? I get my driving licence back next month after a 2-year ban for drink/driving. I no longer drink but I do smoke Marijuana occasionally. I have to be blood tested by a doctor before I finally receive my licence - do you know if they also test for Marijuana and if so would this invalidate my application. I guess what I'm trying to ask is, how long does Marijuana stay in the blood?

A: Marijuana can be detected in blood samples for around few days to a month, depending on what they are looking for (pure drug or metabolites), the sensitivity of the test, how long ago you last used, and how much and for how long you were using.

If your licence is refused on the basis of a positive test for cannabinoids, this should be readily challenged in the courts. There is no evidence that Marijuana causes any significant impairment in driving or risk to road safety (except when used with alcohol), and indeed much evidence suggests that it may contribute to safer driving behaviour, particularly among young men otherwise more prone to driving aggressively.