A recent piece of correspondence from an anonymous reader has provoked a lively discussion among Chemistry World staff. The short email went as follows:

Cannabis buds‘Hello. I’m from Australia and you can’t test for synthetic cannaboids publically my concern is I have a test for it this monday I haven’t had it for 3 and a half weeks but smoked it heavily for 3 months would it still be I’n my system happy to pay money to now this. Thanks.’

We subjected the letter to a sophisticated grammatical analysis of our own devising and can conclude that the author is certainly under the influence of something, although whether it is cannabis, we cannot say for certain. At Chemistry World we are not in the business of doling out advice on how to scupper drug tests, but having said that, it is an interesting question…

As recently as May this year, Chemistry World reported on new urine tests for the synthetic cannabis mimics ‘spice’ and ‘K2’. Like most tests of this sort, they use tandem liquid chromatography – mass spectrometry techniques to quantify metabolites specific to the mimics. New cannabis mimics are appearing rapidly though, and it’s hard to develop and authenticate tests for the new molecules at the same rate. We don’t know what molecules our writer has been using, but it’s unlikely there will be tests for it in routine use by, say, an employer. I asked LGC (formerly the Laboratories of the Government Chemist) if they knew of any accurate tests for these molecules, who informed me they ‘don’t do anything like that’. It seems then, that our hapless correspondent will probably be all right.

What about real cannabis though? Tests for tetrahydrocannabinol, (THC) the active molecular species in cannabis (which we’ve previously covered in our podcast), are well known. So the question arises: how long after you stop smoking (real) cannabis would you pass a drugs test?

Tetrahydrocannabinol (THC)

The answer depends on how the analysis is done. Analysing a urine sample is perhaps the obvious option, but analysing hair and oral fluid is also possible: and the tests give different results. Tiny residues of many chemicals can be found in the hair for long periods – much longer than they would be present in urine, for example – but sensitive analytical methods to detect them are required.

A recent paper reported an accurate method for measuring THC traces in hair. This test also looks for 11-nor-9-carboxytetrahydrocannabinol (9-CTHC), which is important: THC would be found in the hair of someone who has been smoking cannabis, but it might also be found in the hair of someone in the same room as a smoker, who’s never touched the stuff themselves. 9-CTHC is produced from THC during metabolism. That means finding that chemical in someone’s hair is much stiffer proof of cannabis use.

Another paper has been published on the emerging field of testing for cannabis in oral fluids. It’s a much more convenient test, as it doesn’t involve cutting out people’s hair or pre-treating the biological samples to get the drug molecules out. Instead, a swab of saliva can be taken in a few seconds, diluted and analysed by chromatography almost directly.


Unfortunately it’s very hard to give a definitive answer to our enquirer’s question about whether cannabis can be detected in an individual’s system. It depends entirely upon what kind of test is used, how sensitive it is and what samples are taken from the body. Of course it also depends upon how much cannabis has been imbibed. As is so often the case in science, the answer is not as black and white as we might like.

Josh Howgego

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