A drug-DUI case often has no breath-alcohol number that tells the jury where the legal line is. Instead, prosecutors may rely on a blood test showing a prescription medication, marijuana metabolite, stimulant, opioid, or other substance and combine that result with driving behavior and officer observations. The central question is not simply whether a substance was present—it is whether the substance impaired the driver’s ability to safely operate the vehicle.
That difference makes toxicology especially important. Some drugs remain detectable after their impairing effects have faded, lawful prescriptions can appear on a report, and multiple substances can interact in ways that require careful interpretation. Kevin Kennedy defends drug-DUI cases by comparing the laboratory evidence with what the driver actually looked and acted like at the time.
Tennessee DUI Law Covers Legal and Illegal Drugs
Tenn. Code Ann. § 55-10-401 prohibits driving while under the influence of marijuana, a controlled substance, a controlled-substance analogue, a drug, a substance affecting the central nervous system, or a combination of substances when the impairment deprives the driver of the clearness of mind and control necessary for safe operation.
The statute is not limited to illegal drugs. A medication can support a DUI charge even when it was lawfully prescribed if the State proves that it actually impaired driving.
There Is Usually No Simple Per Se Number
Alcohol cases often involve a familiar 0.08% per se threshold. Most drug-DUI cases do not work that way. The mere presence of a drug in blood does not automatically establish that the driver was impaired at the time of operation.
The prosecution generally has to connect the toxicology result to observed impairment, driving behavior, admissions, dosage, timing, or expert testimony.
Marijuana Evidence Can Be Particularly Complicated
THC and its metabolites do not correlate with impairment as neatly as blood-alcohol concentration. A blood result may show recent use, but the significance of the concentration can depend on timing, frequency of use, metabolism, and the particular analyte reported.
The defense should examine whether the laboratory measured active THC, inactive metabolites, or both and whether the State’s expert is making claims that the science actually supports.
Prescription Medication Does Not Equal Guilt
A driver may test positive for benzodiazepines, pain medication, sleep medication, stimulants, or another prescription drug. That can become evidence, but it does not prove unlawful use or impairment by itself.
Pharmacy records, prescribed dosage, tolerance, timing of the last dose, physician instructions, and video of the driver’s behavior may help show whether the medication actually affected safe driving.
Drug Recognition Evaluations and Officer Observations
Police may use specially trained drug-recognition evaluators or rely on observations such as pupil size, pulse, muscle tone, divided-attention testing, speech, or coordination. Those observations should be compared with the video and with alternative medical explanations.
Fatigue, anxiety, neurological conditions, injuries, illness, and prescribed medications can sometimes produce signs that officers interpret as drug impairment.
Blood Testing and Chain of Custody
Drug-DUI cases commonly rely on blood rather than breath testing. The defense should review how the blood was obtained, whether consent or a warrant was valid, who drew the sample, the evidence seals, storage conditions, transfers to the laboratory, and the testing method.
Laboratory reports may identify multiple substances at different concentrations. Each result should be interpreted rather than treated as a single conclusion that the driver was impaired.
Hospital Toxicology Is Not Always the Same as Forensic Toxicology
A hospital may test blood or urine for treatment purposes using methods designed for clinical care rather than criminal prosecution. Screening tests may produce presumptive results that require confirmation.
If the State relies on medical records, the defense should determine what test was performed, whether the result was confirmed, and whether the concentration or substance actually supports the prosecution’s interpretation.
Drug DUI Can Combine With Other Charges
If impairment allegedly causes serious bodily injury or death, the case can escalate into vehicular assault, vehicular homicide, or an aggravated form depending on the facts and prior history.
Possession of drugs or paraphernalia discovered during the stop may also produce separate charges, but those offenses have different elements from DUI.
How Kevin Kennedy Defends DUI With Drugs
Kevin compares the laboratory evidence with the driver’s actual behavior. He reviews body-worn and dash-camera footage, toxicology, prescriptions, pharmacy records, field sobriety testing, drug-recognition evidence, medical conditions, statements, and the legality of the stop and blood draw.
The defense may challenge whether the substance caused impairment, whether the test is reliable, whether the officer’s observations fit the video, or whether another explanation better accounts for the driving. If you are charged with DUI involving drugs in Nashville or Middle Tennessee, contact Kevin Kennedy Law Firm for a free, confidential consultation.
Legal references: Tenn. Code Ann. §§ 55-10-401, 55-10-406, 55-10-408, and 55-10-411. This page is general information, not legal advice.