When most people hear the term “DUI,” they immediately think of alcohol. But in Tennessee, DUI charges are not limited to drinking and driving. A person can also be charged with DUI based on marijuana, illegal drugs, prescription medications, over-the-counter medications, or a combination of substances that allegedly affect their ability to drive safely.
That distinction matters because drug-based DUI cases are often built on different evidence than alcohol-based DUIs. Instead of focusing primarily on a breath test or blood alcohol concentration, the case may turn on whether the State can prove that a particular substance actually impaired the person’s ability to drive.
Because of that, drug DUI cases often require a different defense strategy. Below, we’ll explain the key differences between alcohol-based and drug-based DUIs in Tennessee, why drug evidence can be harder to interpret, and why these cases require specific legal and scientific knowledge.
Tennessee DUI Law Covers Alcohol and Drugs
Tennessee’s DUI law applies to more than alcohol. Under Tennessee Code § 55-10-401, a person may be charged with DUI for driving or being in physical control of a vehicle while under the influence of alcohol, marijuana, a controlled substance, a drug, a substance affecting the central nervous system, or a combination of those substances. The law also includes Tennessee’s “per se” alcohol limit, which makes it unlawful to drive with a blood alcohol concentration of 0.08% or more for most drivers.
In an alcohol-based DUI, the State may focus heavily on a breath or blood alcohol result. In a drug-based DUI, there may not be a simple number that answers the key question. The alleged substance may be marijuana, prescription medication, an illegal drug, or a combination of drugs and alcohol, and each situation can raise different legal, scientific, and factual issues.
The important point is that alcohol DUIs and drug DUIs may fall under the same Tennessee statute, but they are not always proven—or defended—the same way.
The Key Difference Between Alcohol DUIs and Drug DUIs
The biggest difference between an alcohol-based DUI and a drug-based DUI is how the State tries to prove impairment. In many alcohol DUI cases, the prosecution relies on a breath or blood test showing the driver’s blood alcohol concentration. If the result is 0.08% or higher, the State may argue that the number itself violates Tennessee’s DUI law.
Drug DUI cases are usually less straightforward. Unlike alcohol, many drugs do not have a simple legal limit that automatically establishes impairment. Instead, the State often relies on a combination of evidence, including blood test results, officer observations, field sobriety tests, statements made during the stop, driving behavior, and sometimes expert testimony.
That difference matters. A toxicology report may show that a substance was present in someone’s system, but it may not explain when the substance was used, how much was used, or whether it affected the person’s driving.
In other words, alcohol DUI cases often focus on a number. Drug DUI cases often focus on interpretation. That interpretation may involve toxicology, pharmacology, medical history, prescription use, officer training, and the timing of the alleged drug use compared to the time of driving.
A Positive Drug Test Does Not Always Prove Impairment
One of the most important issues in a drug DUI case is that a positive drug test does not automatically prove impairment. A lab report may show that a substance was present in a person’s system, but presence alone is only one piece of the case.
Drug testing can raise complicated timing issues. Some substances may remain detectable after their impairing effects have worn off. Others may appear in a blood test at levels that require expert interpretation. In some cases, the test may detect a metabolite, which can show prior use but may not prove that the person was under the influence while behind the wheel.
This is especially important in cases involving marijuana or prescription medication. A person may have taken medication as prescribed, or may have used a substance earlier, yet the legal issue remains whether that substance affected their ability to drive safely.
A strong defense looks closely at what the toxicology report actually says and what it leaves unanswered. That may involve questions about dosage, timing, tolerance, metabolism, medical conditions, and whether the officer’s observations are consistent with the alleged substance.
Why Prescription Medication and Marijuana DUI Cases Are Different
Drug DUI cases can become especially complicated when they involve prescription medication or marijuana. In both situations, the State may point to the presence of a substance and argue that the person was impaired. But the defense often turns on what that evidence actually proves.
Prescription medication cases are often misunderstood. Many people assume that having a valid prescription prevents a DUI charge, but Tennessee law does not treat a prescription as an automatic defense. The issue is not simply whether the person was legally allowed to take the medication, but whether the medication impaired their ability to safely operate a vehicle. Tennessee law specifically provides that lawful entitlement to use a substance is not, by itself, a defense to DUI.
That does not mean every person who drives after taking medication is guilty of DUI. These cases may require reviewing the type of medication, dosage, timing, side effects, tolerance, and whether the person took the medication as prescribed. They may also involve questions about whether the officer mistook normal side effects, nervousness, fatigue, illness, or a medical condition for impairment.
Marijuana DUI cases raise their own issues. Tennessee’s DUI statute specifically includes marijuana among the substances that can support a DUI charge if it impairs the driver’s ability to safely operate a vehicle. But unlike alcohol, marijuana cases often do not involve a simple legal limit that answers the impairment question. A positive test may raise questions about prior use, timing, active impairment, and whether the person’s driving was actually affected.
That is why prescription medication and marijuana DUIs often require more than simply challenging whether a substance was present. The defense may need to examine whether the State can connect that substance to actual impairment.
How Officers Evaluate Suspected Drug Impairment
Drug DUI cases often depend heavily on what the officer claims to have observed before, during, and after the traffic stop. Unlike many alcohol DUI cases, where the State may rely on a breath test or a blood alcohol number, drug DUI cases often require the officer to interpret signs that may be less obvious and harder to connect to a specific substance.
That makes officer training important. Some officers receive specialized training in drug impairment investigations, such as Advanced Roadside Impaired Driving Enforcement (ARIDE) or Drug Recognition Expert (DRE) training. That training may affect how the officer evaluates things like eye movements, pupil size, balance, coordination, speech, behavior, vital signs, and other physical indicators.
But officer observations are not the same as proof. Many signs associated with impairment can have innocent or medical explanations. Nervousness, fatigue, anxiety, allergies, illness, injuries, poor balance, speech issues, or the stress of a traffic stop may all affect how someone appears on the roadside. Field sobriety tests can also be influenced by weather, lighting, footwear, road conditions, age, weight, or physical limitations.
A proper defense looks at whether the officer’s observations were reliable, whether they matched the alleged substance, and whether alternative explanations were considered. The State should not be allowed to turn assumptions, incomplete observations, or generalized claims about drug impairment into proof beyond a reasonable doubt.
Building a Defense in a Tennessee Drug DUI Case
Because drug DUI cases often rely on interpretation rather than a single number, the defense strategy must be more technical and case-specific. It is not enough to ask whether a substance was found in the person’s system. The real issue is whether the evidence proves impairment.
A strong drug DUI defense often starts with the same foundational questions as any DUI case. Was the traffic stop lawful? Did the officer have probable cause to make an arrest? Were field sobriety tests properly explained and administered? Was there a valid basis for requesting a blood test? Were the person’s rights respected throughout the investigation?
From there, drug DUI cases often require a deeper review of the evidence. The defense may need to examine the blood draw, chain of custody, lab procedures, toxicology report, and whether the test results support the officer’s conclusions. In some cases, the State may have evidence that a substance was present, but far less evidence that it caused impairment.
Drug DUI defense may also involve comparing the officer’s observations to the substance alleged in the case. Different drugs can affect people in different ways, and some symptoms may be caused by medical conditions, prescription medication, fatigue, anxiety, or the stress of the stop itself. If the officer assumed impairment without ruling out other explanations, that assumption may become an important part of the defense.
This is why drug DUI cases require more than general DUI knowledge. A proper defense may involve toxicology, pharmacology, officer training, medical history, prescription records, and the limits of roadside testing. The goal is to separate what the State can actually prove from what it is asking the court or jury to assume.
Charged with a Drug DUI? Talk to a Tennessee DUI Defense Lawyer
Alcohol DUI and drug DUI charges may fall under the same Tennessee DUI law, but they are not always proven the same way—and they should not always be defended the same way. Drug-based DUI cases can involve toxicology, prescription medication, marijuana testing, officer training, medical explanations, and disputes over whether the evidence proves actual impairment.
That is where specific knowledge matters. A positive drug test, roadside observation, or officer assumption may be only one part of the story. The defense must look closely at what the evidence proves, what it does not prove, and whether the State’s conclusions hold up under scrutiny.
If you were charged with a drug-based DUI in Knoxville or East Tennessee, Barnes & Fersten can review the facts of your case and potential defenses. Contact our office today by calling 865-805-5703 or filling out our contact form for a free consultation.
Attorney At Law, Managing Partner
Brandon D. Fersten is an esteemed Knoxville attorney practicing DUI, criminal defense, and juvenile law. Known for his empathetic approach and commitment to his clients, he brings a record of favorable case outcomes including dismissals and not guilty verdicts at jury trials resulting in Brandon being recognized as one of the “Top 40 Under 40” in Criminal Defense, U.S. News’ Best Lawyers: “Ones to Watch,” and Super Lawyers’ “Rising Stars”. Brandon’s professional accolades, combined with his passion for justice, position him as a reliable criminal defense advocate in the East Tennessee legal landscape, including Knox County, Blount County, Sevier County, Loudon County, Roane County, Anderson County, Cumberland County, Hamblen County, Monroe County, and McMinn County.
