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Dr Chip (M.D.)
Dr Chip (M.D.), Doctor
Category: Medical
Satisfied Customers: 29366
Experience:  20 yrs. in practice, includinge surgery, general medicine, addiction medicine and pain.
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Does second hand marijuana smoke show up on a drug test

Customer Question

Does second hand marijuana smoke show up on a drug test?
Submitted: 6 years ago.
Category: Medical
Expert:  Dr Chip (M.D.) replied 6 years ago.
Can you tell me a bit more about the situation and your question?
Customer: replied 6 years ago.
My son has admitted to smoking marijuana with friends and has tested positive with a test. He now says he has given up but still socialises with friends who still smoke. I will test him again. Will the fact that he has been with friends who are smoking show up on his nexr test if he hasn't smoked himself? does a positive test mean he has himself smoked marijuana?
Expert:  Dr Chip (M.D.) replied 6 years ago.
It is possible for second-hand smoke from marijuana to be positive in a drug test, but that usually would be in a situation where the person was in a closed space with heavy smokers, as in a car. You should know, however, that quite often in cases like this, the user will use excuses like this in the hopes that they will be believed. I think, even if he is telling the truth, he's taking a risk of the friends getting busted by the law and then he'll be caught up in the legal system, so, even if you do believe him, you should advise him not to be around anyone in possession of illegal substances. If he tests positive again, there's really no way to tell whether he's actually smoked weed himself or just been around someone who has.
Expert:  Family Physician replied 6 years ago.

There are several published studies that do demonstrate low level positive drug urine and blood drug tests after passive exposure to HIGH levels of marijuana smoke. This is one of those studies:


In this case, individuals who spent time in a small unventilated room in which 16 joints were smoked were found to have low levels of marijuana for short periods of time.


I believe this addresses the issue of how much exposure might be needed.


HOWEVER, this type of exposure is not really realistic to the claim that someone was accidentally exposed without their knowledge (since you would essentially have to be essentially in a complete cloud of smoke). Multiple studies have been conducted with more realistic levels of exposure and they have NOT found positives with a more typical "passive" exposure above the threshold used in your test. This is the most recent of those studies.


You should also know that the home tests can give false positives because of the technology used. To get a valid test, any positive specimen would need to be sent for GC/MS technology.


The other important point, a negative test does NOT mean that your son is not using marijuana or some other illegal drug. With casual use, the drug tests will only identify the drug metabolites for about 3 days. Even with use 3-4 times per day, the test would likely be negative within 5-7 days. Synthetic cannabinoids ("Spice", "K2" or JWH-018) have similar effects to marijuana however they will not be identified by current drug test that are designed to identify THC.

Expert:  Dr Chip (M.D.) replied 6 years ago.
The botXXXXX XXXXXne here is that, if he tests positive again, although it could be a false positive, there would be no way to confirm his alibi unless another, more exact test, were used.
Customer: replied 6 years ago.
What drug testing equipment would you suggest, to ensure my son is clean. i.e not actively smoking.
Expert:  Dr Chip (M.D.) replied 6 years ago.
As the other expert mentioned, having blood or urine sent by a physician to a lab for the GC/MS test would rule out a false positive.
Customer: replied 6 years ago.
what is the GC/MS test?
Expert:  Dr Chip (M.D.) replied 6 years ago.
It's known as gas chromography/mass spectrometry. It is a very detailed test that essentially eliminates the problem with false positives that can be seen with less sophisticated tests such as at home testing.
Dr Chip (M.D.), Doctor
Category: Medical
Satisfied Customers: 29366
Experience: 20 yrs. in practice, includinge surgery, general medicine, addiction medicine and pain.
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