The accuracy of sobriety tests conducted with properly calibrated blood-alcohol concentration (BAC) instruments has been well-established. They're not perfect, but relatively accurate at determining one's level of alcohol intoxication at the time of a traffic stop. But alcohol is just one of countless substances that can cause impairment. Others, such as marijuana, heroin, and even legal prescription painkillers, are much more difficult to measure. Many of these drugs stay in the system much longer than alcohol -- long after they stop causing impairment -- which creates a challenge for police officers trying to determine accurate impairment levels at a precise point in time.
When police are gathering evidence for a driving under the influence charge (DUI), whether its alcohol or some other substance, they look for the byproducts of the ingested substance, called metabolites. DUI charges must be based on evidence, so these metabolites are offered as proof that a certain amount of a given substance was metabolized by the defendant at a given time.
Drug-Related DUIs and Metabolites
Breathalyzer tests don't directly measure the amount of alcohol in one's bloodstream, but rather the concentration of the metabolites alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH). Since alcohol is metabolized at a fairly quick rate -- roughly .016 percent (or one average alcoholic drink) per hour -- this measurement provides a reasonably accurate snapshot of a driver's BAC at the time of a traffic stop.
This is not the case for legal or illegal drugs all of which break down into their own unique metabolites for which there are limited ways to detect and accurately measure. As a result, many states have a "zero tolerance" or "per se" approach to non-alcohol DUI charges. This means that a motorist who is not impaired at the time of a traffic stop still may be found guilty of driving under the influence of drugs if the metabolites (either above a certain level or any detectable level, depending on state law) are present at the time of the stop.
Driving Under the Influence of Marijuana
Marijuana, the most widely used recreational drug in the U.S. besides alcohol, is perhaps the most difficult to measure in the context of a DUI investigation. And while many states will arrest DUI suspects who have any measurable level of THC (the psychoactive component of marijuana) in their blood, lawmakers in states that have legalized the recreational or medical use of the herb are struggling to curb "stoned driving" while also respecting the rights of their residents.
This is compounded by the fact that marijuana metabolites remain in the bloodstream (as measured in the urine) for roughly one week after ingestion, even though the high itself may only last a few hours. This means that the presence of marijuana metabolites in a urinalysis would need supporting evidence from a drug recognition expert, such as erratic lane changes, bloodshot eyes, etc. A research report published by the Connecticut General Assembly about DUI urine tests for marijuana explains how Connecticut courts limit the use of urine tests in such cases.
It should also be pointed out that the legal use of medical marijuana is no defense against DUI charges if it causes impairment or if the state has a "per se" DUI law.
State Approaches to Measuring Controlled Substance Use by Motorists
As mentioned above, some states take a zero tolerance approach to impaired driving, whether it's alcohol or some other substance, regardless of whether the driver was actually impaired at the time of the stop. This means any detectable drug metabolite revealed through urine or blood screening can result in a DUI charge in certain states, typically with other evidence of impairment. Blood draws are much less common, considering how intrusive they are, but state courts are divided in how police may compel a blood draw after a DUI arrest.
The list of states with zero tolerance drugged driving laws include (but are not limited to) Iowa, Georgia, Arizona, and Michigan (the latter two states allow for the medical use of marijuana). Other states have a more nuanced approach to the assessment of drug impairment, often through established per se limits for certain drugs. These states include (but are not limited to) the following:
Many states, including California, have two separate charges related to DUI. One charge is for having a BAC higher than 0.08 percent -- or, generally, exceeding the limit for alcohol metabolites -- and the other pertains to other evidence of impairment. In other words, you can be charged with a DUI in California for showing clear signs of impairment (regardless of BAC or metabolite content), having a BAC higher than 0.08 percent (or a detectable amount of drugs), or both. While there is no immediate breath test for drugs other than alcohol, either blood or urine may be used to screen for most drugs.
Get a Free Legal Analysis of Your DUI-Drugs Case
If you are determined to exceed a certain level of drug metabolites, a DUI charge may be leveled against you regardless of whether you were actually impaired at the time. But there are ways to defend against these types of charges, including the integrity of the metabolite test itself. Get a head start today by having a defense attorney evaluate your case at absolutely no charge.