Testing for Marijuana

In 2011, 57 percent of U.S. businesses required prospective employees to pass a drug test, according to a poll conducted by the Society for Human Resource Management. Drug tests detect the five metabolites that THC breaks down into before passing into urine. Detectable amounts can remain in the system for several days or weeks following use, depending on potency and amount consumed.

One of the most common tests is the immunoassay, in which urine is mixed with a solution containing an antibody specific to certain metabolites, typically tagged with fluorescent dye or a radioactive substance. The amount of fluorescent light or radioactivity indicates the concentration of metabolites in the sample.

Gas chromatography/mass spectrometry may also be used to test for THC metabolites.

Marijuana Potency

Whether marijuana is more potent today than it was 30 or 40 years ago is at the center of much debate. The U.S. federal government has said that the levels of potency have risen anywhere from 10 to 25 times since the 1960s. Is this a myth or reality?

"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."

That's from Alan Leshner, the Director of the National Institute on Drug Abuse, while he was testifying in front of the U.S. House Subcommittee on Crime in 1999.

Those who support the legalization of marijuana say that the data are skewed because testing was only performed on marijuana of specific geographic origins in the 1960s and 1970s, and therefore isn't representative of marijuana potency overall. Officials obtained the samples from a type of Mexican marijuana that is known to contain low levels of THC -- 0.4 to 1 percent [source: Kuhn et al.]. When these levels are compared to other types of marijuana, it looks as if potency levels have risen in the last 30 years.

Typical THC levels, which determine marijuana potency, range from 0.3 to 4 percent. However, some specially grown plants can contain THC levels as high as 25 percent, leading to a call by some users for producers to put out mellower Mary Jane [source: Marris]. Several factors are involved in determining the potency of a marijuana plant, including:

  • Growing climate and conditions
  • Plant genetics
  • Harvesting and processing
  • Desire by small growers to maximize profit

The time at which the plant is harvested affects the level of THC. Additionally, female varieties have higher levels of THC than male varieties.

As a cannabis plant matures, its chemical composition changes. During early development, cannabidiolic acid is the most prevalent chemical. Later, cannabidiolic acid is converted to cannabidiol, which is later converted to THC when the plant reaches its floral maturation.

To determine the average potency levels of marijuana, researchers need to examine a cross section of cannabis plants, which wasn't done in the 1960s and 1970s. This makes it difficult to make accurate comparisons between the THC levels of that period and the THC levels of today. Moreover, establishing a clear relationship between THC levels and impairment is not as straightforward as with, say, blood alcohol content.