It has been widely claimed that the dosage levels of psychoactive drugs from second-hand marijuana smoke is insufficient to have an effect - i.e. to give a "contact high"

  • Opposing Views

    The notion of the “contact high” is a myth. In order to get “high”, the amount of active THC in your bloodstream has to spike.

  • The Emergent Fool

    Many users, and friends of users of marijuana report experiencing a “contact high.” That is, they purport to experience some of the effects of marijuana simply by being in contact with or around those using marijuana. Virtually all users wrongly attribute this experience to the inhalation of second hand smoke. This is unlikely for a number of reasons. Since exhaled smoke is virtually devoid of psychoactive substances and is widely dispersed in the surrounding air, it is not possible for one to inhale even a small fraction of a working dose. Additionally, many, including noted pharmacologist and psychedelic researcher Alexander Shulgin, report similar experiences involving drugs that are not inhaled, indicating that these effects are due to different biological processes.

However, this doesn't tally with the anecdotes and claims of others:

  • Wiki Answers

    I'm allergic to marijuana, so I've gotten coughing and asthma from second-hand smoke, and also I get painful sores on my head when the neighbor smokes it. It gives me mood swings, then I smell her pot half an hour later. One time I was very, very tired from the marijuana smoke coming in, then when it wore off, my heart beat really fast for a couple of hours

  • Wiki Answers

  • Oregon State University Student Health Services

    Exposure to second hand smoke can cause a drug test to be positive. A "contact high" indicates that THC has entered the blood stream where it is then detectable by drug analysis.

  • secondhandmarijuana.com

    However, despite there being evidence that the second hand marijuana smoke can produce a very small high in people, there is no evidence that the smoke can cause a failed drug test.

My personal experience is that even walking a few meters behind a person smoking a joint outdoors is sufficient to give me symptoms 20 minutes later, such as a strange feeling in the head area, slight alteration of taste, vivid dreams, slight insomnia, and an infuriating loss of higher mathematical ability.

What does the evidence say? Can the miniscule dosages inhaled this way lead to real signs of a small marijuana high, or are these symptoms of a nocebo effect?

  • As is, this uestion may be more suitable for Biology as it seems to be about a non-notable claim. We require claims to be believed as true by a large number of adults -- whereas there seems to be no one accepting this as fact. Can you provide notability examples?
    – Sklivvz
    Commented Sep 1, 2012 at 7:09
  • @Sklivvz: You can find literature on this--- drug testing sets a limit at 50 ng because levels as high as 4 ng are detected from passive smoking, like if you are at an indoor party where others are smoking marijuana. But this level is small, and is considered non-psychoactive by drug-users and most websites. I expect 1ng from an exposure of the sort I describe. Is there any evidence that 1ng is psychoactive. These are ng per standard unit blood, I don't remember the standard unit. As for "large number of adults", google "contact high".
    – Ron Maimon
    Commented Sep 1, 2012 at 7:40
  • 1
    Looking on various drug forums, I came across a fellow who said he wasn't smoking but tested at 61 ng (and 14ng a few days later) from passive smoke! He was around heavy smokers, but didn't inhale himself. This is the type of levels one would expect from a reasonably regular marijuana smoker. I would like to see a good study showing whether contact high from secondhand inhalation is a myth, or whether the myth is that it is a myth.
    – Ron Maimon
    Commented Sep 1, 2012 at 8:27
  • I went to try to save it this post, but finding some actual examples of the claim, and citing them. My initial search cam up with nothing but debunking! (And Wikipedia suggesting the term "contact high" is being used incorrectly when talking about inhaling second hand marijuana smoke.) I figured I wasn't making the Internet more awesome with just another identical article, so I didn't bother chasing further.
    – Oddthinking
    Commented Sep 1, 2012 at 8:32
  • Note the three different claims now: in the presence of people who are high leads to high, in the presence of second-hand smoke leads to high, in the presence of second hand smoke leads to positive drug test.
    – Oddthinking
    Commented Sep 1, 2012 at 8:34

1 Answer 1


I had to dig a little, but I believe I have an answer for you, and the answer is: Yes, but how quickly depends on the user.

I'm drawing from this study from the British Journal of Anesthesia. According to the study, your average modern joint is going to have between 60mg and 150mg of THC (which mirrors information gathered by the National Highway Traffic Safety Administration), with upwards of 300mg of THC if there's hashish oil involved.

From the study: THC 2.5mg in a cigarette is enough to produce measurable psychological and physical effects in occasional cannabis users.

However (emphasis mine): Approximately 50% of the THC and other cannabinoids present in a cannabis cigarette enter the mainstream smoke and are inhaled. The amount absorbed through the lungs depends on the smoking style. In experienced smokers, who inhale deeply and hold the smoke in the lungs for some seconds before exhaling, virtually all of the cannabinoids present in the mainstream smoke enter the bloodstream.

So if it's an inexperienced toker and they're taking quick, short puffs, then it's plausible that you could (assuming you were inhaling significant amounts of the exhaled smoke) get a contact high that only slightly lags behind the person smoking.

At the same time, if you're in the same room as an experienced user who holds the breath in for a couple of seconds, it might take you a while. It will eventually happen, though.

To address the comment below (emphasis mine):

Independent Drug Monitoring in the UK found that 15mg of THC would produce serum levels of between 14-42ng/ml THC. Sobriety tests were consistently failed when serum levels of THC achieved 25ng-30ng/ml.

4.2| Positive tests for cannabinoids in urine may also occur as a result of passive smoking, with cannabinoid (THC-acid) levels of over 20ng/ml detectable in one case 4 days after passive exposure. It was concluded that presence of cannabinoids in urine or blood is not unequivocal proof of active cannabis smoking... Magerl et al found THC-acid levels of up to 30-50ng/ml from passive-exposed subjects, and recommended a threshold of 65ng/ml to differentiate between active and passive smoking of cannabis.

Yes, experiments have confirmed cases where second-hand exposure to marijuana smoke produces blood-serum levels equivalent to or greater than levels which are associated with impairment.

You can get a contact high from second-hand marijuana smoke.

Addendum: I cannot find an experiment done outdoors. I think the implausibility is enough to deter researchers. The closest lower limit I could find for impairment was 2-5ng here.


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