Pg/mg stands for picograms per milligram, and it’s the standard unit of measurement used in hair drug testing. A picogram is one trillionth of a gram, so this unit expresses an incredibly tiny concentration of a drug substance found within a small weight of hair. If your drug test results show numbers in pg/mg, you almost certainly had a hair follicle test rather than a urine or saliva test.
What Picograms Per Milligram Actually Measures
When a lab analyzes a hair sample, it dissolves or breaks down the hair and then measures how much of a drug or its byproducts are trapped inside. The result tells you how many picograms of a substance were found in each milligram of hair. Think of it like parts per billion: the drug concentrations in hair are extremely small compared to what shows up in urine or blood, so the unit has to be correspondingly tiny.
This unit exists because drugs circulating in your bloodstream get incorporated into the hair shaft as it grows. Head hair grows about half an inch per month, so a standard 1.5-inch sample covers roughly 90 days of drug exposure. The lab weighs the hair, processes it, and reports the concentration in pg/mg. That number is then compared against an established cutoff to determine whether the test is positive or negative.
How pg/mg Differs From Urine Test Units
Urine drug tests report results in nanograms per milliliter (ng/mL), which is a thousand times larger than a picogram. The Department of Transportation, for example, sets urine screening cutoffs at 50 ng/mL for marijuana metabolites and 150 ng/mL for cocaine metabolites. Hair test cutoffs, by contrast, sit at levels like 1.0 pg/mg for marijuana metabolites and 300 to 500 pg/mg for cocaine. These aren’t directly comparable because one measures concentration in a liquid and the other in a solid, but the key difference is that hair testing detects drug use over a much longer window (up to 90 days) while urine typically covers one to seven days.
Oral fluid tests also use ng/mL and detect use within roughly 5 to 48 hours. So when you see pg/mg on a result, it’s telling you something fundamentally different from those other tests: not whether you used a substance recently, but whether you used it at any point over the past three months.
Cutoff Levels by Drug
A result below the cutoff is reported as negative. A result at or above it triggers a confirmation test using more precise equipment. Cutoffs vary slightly between laboratories, but the most common thresholds look like this:
- Marijuana (THC metabolite): 1.0 pg/mg for screening, with confirmation as low as 0.1 to 0.3 pg/mg depending on the lab
- Cocaine and metabolites: 300 to 500 pg/mg for both screening and confirmation
- Amphetamines (including MDMA): 300 to 500 pg/mg
- Opiates (morphine, codeine, hydrocodone, oxycodone): 300 to 500 pg/mg
Notice that the marijuana cutoff is dramatically lower than the others. That’s because THC and its byproducts incorporate into hair at much lower concentrations than stimulants or opiates. Cocaine, by contrast, binds to hair readily. Positive cocaine results commonly range from 100 to over 21,000 pg/mg, with an average around 2,600 pg/mg in confirmed positive samples.
What Your Number Means in Practice
If your result simply says “positive” or “negative,” the lab has already compared your pg/mg value against the cutoff. Some reports do include the actual numerical value, though, and people naturally want to know what a higher or lower number indicates.
Generally, a higher pg/mg value suggests more frequent or heavier use, though the relationship isn’t perfectly precise. Research on heroin users, for instance, shows an apparently linear relationship between how often someone reports using and the concentration of morphine and its byproducts found in their hair. For methamphetamine, low-dose users showed hair concentrations of 600 to 3,500 pg/mg, while high-dose users showed 1,200 to 5,300 pg/mg. So the numbers do scale with use, but individual variation in hair growth rate, thickness, and melanin content makes it impossible to calculate an exact dose from a pg/mg reading.
Factors That Can Change Your pg/mg Level
Hair color and cosmetic treatments can influence results. Darker hair tends to bind certain drugs more readily because melanin plays a role in trapping drug molecules. This doesn’t mean dark-haired people are more likely to test positive if they haven’t used, but it can affect the concentration detected among people with similar use patterns.
Chemical treatments like bleaching, perming, and dyeing can reduce drug concentrations in hair, sometimes significantly. In vitro studies on alcohol markers have shown that aggressive bleaching can completely eliminate detectable levels of certain substances from the hair matrix. This is why labs sometimes note whether a sample shows signs of cosmetic treatment, and it’s one reason confirmation testing uses more sensitive methods. Repeated harsh treatments could, in theory, push a borderline result below the cutoff.
Body hair can also be tested when head hair is unavailable, but body hair grows at different rates and may represent a longer, less defined detection window than the standard 90-day estimate for head hair.
Why Hair Tests Use Such a Tiny Unit
The reason pg/mg exists as a unit comes down to biology. Only a tiny fraction of the drugs in your bloodstream actually end up embedded in your hair. A person with a blood cocaine level measured in milligrams per liter might have a hair cocaine level of 10,000 pg/mg or more, but those are fundamentally different scales. Postmortem studies comparing drug levels in organs versus hair illustrate this clearly: someone with a blood oxycodone level of 0.066 mg/L had a hair oxycodone level of 2,079 pg/mg. The numbers look large, but a picogram is so small that even thousands of them per milligram represent a vanishingly tiny amount of substance.
For practical purposes, all you need to know is that pg/mg is the standard yardstick for hair drug testing. If your result is below the cutoff for the substance being tested, it’s reported as negative. If it’s above, the lab runs a second, more specific confirmation test before finalizing the result as positive.