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"There's no question that marijuana, today, is more potent than the marijuana in the 1960s. However, if you were to look at the average marijuana potency which is about 3.5 percent, it's been relatively stable for the last 20 years. Having said that, it's very important that what we have now is a wider range of potencies available than we had in the 1970s, in particular," Director of the National Institute on Drug Abuse Alan Leshner said in 1999 while testifying in front of the U.S. House Subcommittee on Crime.

Source How stuff Works

I heard this claim again today that the marijuana today is 10x more potent than it was in the 60's. The claim was from an unnamed law enforcement union official (presumably from California) about why marijuana should not be legalized.

How stuff works came to the conclusion that it would be difficult to tell if it was more potent today. But with a claim of 10x that should be more refutable or verifiable. Is there any hard data anywhere to support or refute this or is this just a made up number?

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Summary: I don't have evidence for a 10x increase since the 1960s. I do for an increase from 1.5% THC to 8.8% THC (i.e. fivefold) from 1980-2008.

The University of Mississippi have a long-running "Potency Monitoring Project".

Potency trends of delta9-THC and other cannabinoids in confiscated marijuana from 1980-1997, ElSohly MA, Ross SA, Mehmedic Z, Arafat R, Yi B, Banahan BF 3rd., J Forensic Sci. 2000 Jan;45(1):24-30.

They analysed over 35,000 confiscated "cannabis preparations" over a period of 18 years, and found a rise in potency.

The potency (concentration of delta9-THC) of marijuana samples rose from less than 1.5% in 1980 to approximately 3.3% in 1983 and 1984, then fluctuated around 3% till 1992. Since 1992, the potency of confiscated marijuana samples has continuously risen, going from 3.1% in 1992 to 4.2% in 1997. The average concentration of delta9-THC in all cannabis samples showed a gradual rise from 3% in 1991 to 4.47% in 1997. Hashish and hash oil, on the other hand, showed no specific potency trends. Other major cannabinoids [cannabidiol (CBD), cannabinol (CBN), and cannabichromene (CBC)] showed no significant change in their concentration over the years.

More recently, they updated this to show (now over 46,000 samples):

The data showed an upward trend in the mean D9-tetrahydrocannabinol (D9-THC) content of all confiscated cannabis preparations, which increased from 3.4% in 1993 to 8.8% in 2008. Hashish potencies did not increase consistently during this period; however, the mean yearly potency varied from 2.5–9.2% (1993–2003) to 12.0–29.3% (2004–2008). Hash oil potencies also varied considerably during this period (16.8 ± 16.3%). The increase in cannabis preparation potency is mainly due to the increase in the potency of nondomestic versus domestic samples.

Potency Trends of delta9 THC and Other Cannabinoids in Confiscated Cannabis Preparations from 1993 to 2008, Zlatko Mehmedic; Suman Chandra; Desmond Slade; Heather Denham; Susan Foster Amit S. Patel; Samir A. Ross; Ikhlas A. Khan; and Mahmoud A. ElSohly. J Forensic Sci, September 2010, Vol. 55, No. 5, doi: 10.1111/j.1556-4029.2010.01441.x

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    +1 - This is actually much more that I expected. I doubted that that there would be anything that showed it even doubled.
    – Chad
    Oct 20, 2011 at 12:59
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    @Chad I presume these are averages. I'd guess that the most potent strains have not increased very much but that they are more commonly available. I've read literature that referred to "Colombian strength" in past but now such designations are meaningless. Michael Pollan has an interesting part of the book "The Botany of Desire" where he discusses how law enforcement pushing growers indoors has vastly improved the quality of cannibus and claims that the best horticulturalists of our era are growing cannibus.
    – JimmyJames
    Jun 15, 2016 at 21:32

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