The question of how much cannabis equates to a specific blood concentration is complex, yet this technical measurement, expressed in nanograms per milliliter (ng/mL), is used as a forensic and legal standard. Tetrahydrocannabinol (THC), the primary psychoactive compound in cannabis, is measured in the bloodstream to determine recent use and potential impairment. Understanding this measurement requires looking at the extremely small scale of the unit, its legal application, and how the body processes the compound. The concentration of 5 ng/mL has become a frequently discussed threshold in public policy, despite the wide variability in how individuals reach and clear this level.
What is a Nanogram Per Milliliter?
A nanogram per milliliter (ng/mL) is a unit of concentration used in medicine and toxicology to measure a minute amount of a substance within a liquid. A nanogram (ng) is one billionth of a single gram. A milliliter (mL) is a small volume of liquid, approximately one-fifth of a teaspoon. Therefore, ng/mL represents the number of nanograms of a substance, such as THC, found in every milliliter of blood or other bodily fluid.
This measurement is precise because the psychoactive effects of THC are triggered by very low concentrations in the bloodstream. For comparison, the measurement of 5 ng/mL means that five billionths of a gram of THC are present in one milliliter of a person’s blood sample.
The 5 ng/mL Legal Standard
The concentration of 5 ng/mL is often adopted as a legal limit for driving under the influence of cannabis in several jurisdictions. When a law sets a specific blood concentration as automatic evidence of impairment, it is known as a per se limit. In states with this standard, exceeding 5 ng/mL of active THC can result in a charge of impaired driving, regardless of behavioral evidence.
Other jurisdictions, such as Colorado, use a permissible inference standard. This means a blood concentration of 5 ng/mL or higher allows a jury to infer impairment, but it is not automatic proof. This distinction is significant because, unlike alcohol, there is no scientific consensus that 5 ng/mL of THC universally correlates with a specific level of driving impairment. Critics argue that a person’s actual impairment does not consistently align with this specific blood concentration, making the 5 ng/mL threshold controversial.
How Consumption Translates to Blood Concentration
The amount of cannabis required to reach 5 ng/mL depends heavily on the method of consumption, due to differences in how the body absorbs THC.
Inhalation methods, such as smoking or vaping, lead to a rapid spike in blood THC concentration, often peaking within minutes. Even a single puff can result in concentrations significantly higher than 5 ng/mL, sometimes reaching 46 to 188 ng/mL within ten minutes. Following inhalation, the concentration drops quickly as the compound is distributed to tissues. This rapid clearance means blood levels are often below 5 ng/mL within one to three hours after smoking, though impairment may still be present.
In contrast, consuming edibles leads to a much slower absorption rate because the THC must pass through the digestive system and liver. This results in a delayed and lower peak concentration, typically occurring one to five hours after ingestion.
While the peak is lower, a dose of approximately 20 mg of THC can still result in a maximum concentration ranging between 4.4 and 11 ng/mL. The lower absorption rate means reaching the 5 ng/mL threshold takes longer, and the level remains elevated for a more prolonged period compared to inhalation. This difference in pharmacokinetics—the process of absorption, distribution, metabolism, and elimination—explains why it is impossible to equate a single quantity of cannabis with a fixed blood level.
Individual Variables Impacting THC Levels
Individual physiological factors further complicate the wide variation in blood THC levels.
A person’s metabolic rate, influenced by genetics and liver function, dictates how quickly THC is processed and eliminated from the bloodstream. Those with a faster metabolism clear THC more rapidly, leading to a shorter detection window and lower concentrations over time.
A person’s history of cannabis use is another major variable, as frequent users develop tolerance and metabolize THC more efficiently than infrequent users. Because THC is highly fat-soluble, an individual’s Body Mass Index (BMI) also plays a role, as THC is rapidly taken up and stored in fatty tissues.
The stored THC is slowly released back into the bloodstream over time, which complicates the interpretation of low blood concentrations in chronic users. For these users, a blood test may detect low levels of active THC, sometimes less than 1 ng/mL, even without recent consumption. While the active THC level is low, the concentration of the non-psychoactive metabolite, carboxy-THC, may remain high. These biological factors mean the same dose of cannabis can produce vastly different blood concentrations in different people, making the 5 ng/mL standard a highly individualized measure.