The Presence of a Drug Doesn’t Prove Intoxication.

A DWI arrest involving drugs is usually more complex than a standard alcohol case. Still, a positive drug test or a substance found in a blood draw doesn’t mean the State can prove its case.

Unlike alcohol, Texas doesn’t establish a legal limit for drugs. There is no threshold number, no 0.08 equivalent, and no single result that proves intoxication. The presence of a substance and the existence of intoxication are two different things. That distinction drives the defense in every Drug DWI case we handle.

At Denuna & Westbrook, PLLC, we represent individuals facing Drug DWI allegations throughout Houston and the Gulf Coast region. We examine whether the evidence supports the prosecution's position and pursue every available defense from the investigation through trial.

How Drug DWI Cases Differ from Alcohol DWI Cases

Drug DWI cases may involve marijuana, prescription medications, controlled substances, over-the-counter medications, or a combination of multiple substances. What they all have in common is the absence of a numerical standard like a DWI involving alcohol.

Without a threshold number to anchor their case, prosecutors rely on a combination of officer observations, field sobriety tests, toxicology reports, Drug Recognition Expert evaluations, witness statements, and driving behavior. Each piece of that evidence can be examined, challenged, and contested individually.

The ALR Process Applies to DWIs Involving Alcohol and Drugs

A Drug DWI arrest triggers an Administrative License Revocation (ALR) proceeding independent of the criminal case, just like a DWI involving alcohol allegations. In most situations, including breath test failures and test refusals, the officer issues a Notice of Suspension at the scene, and you have 15 days to request an ALR hearing. The exception is a consensual blood draw, where the clock starts when DPS mails a notice after receiving the lab results. Missing these deadlines leads to an automatic suspension regardless of how the case is resolved.

What the State Has to Establish in a Drug DWI Case

Under Texas Penal Code Section 49.04, the State must prove that a person operated a motor vehicle in a public place while intoxicated. In Drug DWI cases, prosecutors focus on two of the three definitions of intoxication:

• Not having the normal use of mental faculties because of a drug, controlled substance, dangerous drug, alcohol, or a combination of substances
• Not having the normal use of physical faculties because of a drug, controlled substance, dangerous drug, alcohol, or a combination of substances

Because there’s no legal limit for drugs, the State has to prove that a person’s mental or physical faculties were affected to the degree required by law, not merely that a substance was present. That’s a meaningful distinction, and it’s where the defense begins.

The Limitations of the State’s Evidence

DWI cases involving drugs rarely hinge on a single piece of objective evidence. Instead, prosecutors combine several categories of circumstantial evidence and ask a jury to connect the dots and draw a conclusion about intoxication. Each category has real limitations. Finding them, challenging them, and making sure the jury understands them is how these cases are won.

Officer Observations

Drug DWI investigations often begin with an officer’s observations about a driver’s appearance, behavior, or mannerisms: red eyes, slow speech, delayed responses, nervousness, confusion, or unsteady movement. Those observations don’t exist in isolation. Fatigue, illness, allergies, stress, anxiety, prescription medications, and medical conditions can all produce similar signs. An officer’s belief that a driver appeared intoxicated doesn’t establish the legal definition of intoxication.

Standardized Field Sobriety Tests

Field sobriety tests are used in Drug DWI investigations, but these tests were developed specifically for alcohol cases. Their accuracy in that context is already subject to challenge, and when applied to drug-related impairment, the scientific foundation is even more questionable. Performance is also influenced by age, fatigue, medical conditions, physical limitations, injuries, environmental conditions, weather, footwear, and nervousness. Because these tests anchor the prosecution's case, the administration and interpretation of every test deserves scrutiny.

Drug Recognition Expert Testimony

In some investigations, law enforcement relies on officers trained as Drug Recognition Experts, commonly referred to as DREs. Prosecutors present DRE testimony as powerful evidence of intoxication. But DRE evaluations don’t directly measure intoxication. They involve a series of observations, tests, and interpretations that allow the officer to offer an opinion about possible drug-related impairment. Like any opinion evidence, those conclusions can be challenged, and the underlying observations and assumptions supporting them deserve independent examination.

Toxicology Testing

A positive toxicology result may indicate a substance was present in the body, but it doesn’t establish the substance caused intoxication at the time of driving. Many substances remain detectable long after their effects have ended. A lab report may confirm the presence of a drug, yet leaves key questions unanswered: when was the substance consumed, how did it affect this particular person, and was intoxication present while the person was behind the wheel? The existence of a positive test is one part of the analysis, not the conclusion.

The Drug Recognition Expert Evaluation: What It Is and Where It Falls Short

When a driver is suspected of drug impairment and a breath test rules out alcohol as the primary cause, law enforcement may call in a Drug Recognition Expert, a specially trained officer who conducts a 12-step evaluation designed to identify which category of drug is causing the impairment. That evaluation and the officer's resulting opinion often become the centerpiece of the prosecution's case.

Understanding what the DRE evaluation actually involves, and where it's most vulnerable, is essential to defending against it.

The 12 Steps of the DRE Evaluation

The DRE protocol follows a standardized sequence:

• Step 1: Breath alcohol test, to rule out or quantify alcohol as the cause of impairment
• Step 2: Interview of the arresting officer, reviewing observations and the circumstances of the stop and arrest
• Step 3: Preliminary examination, including pulse, general demeanor, speech, and pupil size in ambient light
• Step 4: Eye examination, including HGN, Vertical Gaze Nystagmus (VGN), and Lack of Convergence (LOC) tests
• Step 5: Divided attention tests, the same Walk-and-Turn and One-Leg Stand used in alcohol DWI cases
• Step 6: Vital signs examination, including pulse rate, blood pressure, and body temperature
• Step 7: Dark room examination, measuring pupil size under three lighting conditions using a pupilometer
• Step 8: Examination of muscle tone, checking for rigidity or flaccidity associated with certain drug categories
• Step 9: Examination for injection sites, looking for evidence of intravenous drug use
• Step 10: Statements and other observations, questioning the driver about drug use
• Step 11: Opinion of the evaluator, the DRE forms a conclusion about whether the driver is impaired and which drug category is responsible
• Step 12: Toxicological examination, a blood, urine, or saliva sample is collected to attempt to confirm the DRE's opinion

What the DRE Evaluation Can't Actually Tell You

The DRE evaluation is presented to juries as expert science. The reality is more complicated.

The DRE renders an opinion about drug category: stimulants, depressants, hallucinogens, cannabis, opiates, inhalants, or dissociative anesthetics. That opinion is not a medical diagnosis. It's a trained officer's interpretation of a series of physiological observations compared against a symptom matrix. Different drugs in the same category produce different effects. Different people react differently to the same drug. And the relationship between the presence of a substance and actual impairment, the thing the State must prove, is not resolved by the DRE protocol.

The validation research behind the DRE program has significant methodological limitations. The original LAPD validation study was not double-blind. Subsequent studies have produced mixed results on whether DRE evaluations reliably identify impairment rather than simply detecting the presence of a substance.

Where the Defense Focuses in DRE Cases

The most productive defense issues in DRE cases involve:

• Whether the officer's training and certification meet the required standards
• Whether every step of the 12-step protocol was followed correctly and documented
• Whether the physiological indicators observed have alternative explanations, including medical conditions, fatigue, anxiety, or lawful prescription drug use
• Whether the toxicology results actually support the DRE's opinion about drug category and impairment
• Whether the blood, urine, or saliva sample was properly collected, preserved, and analyzed
• Whether the presence of a substance in the toxicology results establishes impairment at the time of driving

A DRE opinion is exactly that: an opinion. It can be challenged, cross-examined, and in some cases, excluded. The strength of a drug DWI case built on DRE evidence depends heavily on how thoroughly that evidence is examined.

Prescription Medications and Marijuana

Can You Be Charged with DWI for Prescription Medication?

Yes. Many Drug DWI arrests involve medications lawfully prescribed by a physician, including sleep medications, anti-anxiety medications, pain medications, stimulants, and other prescription drugs. A valid prescription doesn’t prevent a DWI charge. At the same time, a drug appearing in a toxicology report doesn’t establish intoxication. The central question is whether the medication caused a loss of normal mental or physical faculties while driving.

Can You Get a DWI for Marijuana in Texas?

Yes. Texas has no legal THC limit comparable to the 0.08 alcohol standard. Marijuana-related DWI cases rely on officer observations, field sobriety testing, toxicology evidence, Drug Recognition Expert testimony, and driving behavior. THC can remain detectable long after any intoxicating effects have ended, which means a positive result doesn’t answer the legal question. Prosecutors must still prove that the driver lacked the normal use of mental or physical faculties at the time of driving.

The Investigation Is the Starting Point

A Drug DWI case doesn’t begin with the toxicology report. It begins with a traffic stop. The legality of the stop, the detention, the arrest, the collection of evidence, and the testing procedures can all become issues. If constitutional violations occurred, certain evidence may be subject to challenge or exclusion.

When the evidence is fully examined, issues can emerge around timing, alternative explanations, testing limitations, officer assumptions, and the scientific reliability of the conclusions being drawn. The existence of a substance doesn’t resolve those questions. It raises them.

A Defense That Understands and Challenges the Science

Drug DWI cases involve toxicology reports, blood testing, Drug Recognition testimony, scientific literature, and competing interpretations of laboratory evidence. Challenging that evidence effectively requires understanding it at a level most defense attorneys don’t reach.

Chris Denuna is a Forensic Lawyer-Scientist. This designation reflects his advanced training in the scientific and technical issues that arise in DWI prosecutions, including the science behind drug impairment evaluations, the pharmacokinetics of different drug categories, and the limitations of the DRE protocol. We use this knowledge to effectively evaluate toxicology evidence, challenge DRE opinions, and examine laboratory methodology in ways that go beyond a standard legal review.

Denuna & Westbrook has defended Drug DWI cases involving marijuana, prescription medications, controlled substances, and combinations of multiple substances. The approach stays consistent: examine what the evidence actually shows, challenge what doesn’t hold up, and hold the State to its burden of proof on every element.

Frequently Asked Questions — Drug DWI in Texas

Can I be convicted simply because drugs were found in my system?

No. The presence of a substance doesn’t establish intoxication. The State must prove that you lacked the normal use of your mental or physical faculties because of the substance involved.

Is there a legal limit for drugs like there is for alcohol?

No. Texas doesn’t establish a fixed legal limit for drugs. Prosecutors rely on observations, opinions, and other evidence to establish intoxication.

What does a Drug Recognition Expert actually do?

A DRE conducts a series of observations and evaluations designed to help law enforcement identify possible drug-related impairment. Their conclusions are opinion evidence that can be challenged at trial.

What if my blood test shows drugs in my system?

A positive result may establish that a substance was present. It doesn’t establish intoxication at the time of driving, which is what the State must prove.

Are Drug DWI cases harder to prove?

They often involve scientific and evidentiary issues that don’t exist in alcohol-related cases. Because there’s no clear numerical standard, prosecutors rely on observations, opinions, and circumstantial evidence, all of which can be scrutinized and challenged.

Contact a Houston Drug DWI Lawyer Today

Drug-related DWI allegations involve toxicology testing, blood evidence, Drug Recognition Expert testimony, and scientific questions that rarely arise in standard alcohol cases. A positive test result is not the end of the analysis.

Denuna & Westbrook, PLLC represents individuals facing Drug DWI allegations throughout Houston and the Gulf Coast region, including Harris County, Fort Bend County, Montgomery County, Galveston County, Brazoria County, and Waller County.