Medical Conditions That Mimic Impairment: Defending Texas DWI Cases

A DWI arrest does not always start with someone being “drunk.” In Texas, the legal definition of intoxication includes not having the normal use of mental or physical faculties due to alcohol, drugs, or other substances, and it also includes an alcohol concentration of 0.08 or more.

That matters because some DWI cases are not alcohol cases at all. Some are built on an officer’s observations that look like impairment: slow responses, balance issues, confusion, slurred speech, or unusual behavior. Those observations can be caused by medication side effects, fatigue, and medical conditions that have nothing to do with intoxication.

My name is Paul Key. I defend DWI cases in McKinney and across Collin County and Dallas County. When a case involves medical or fatigue explanations, the defense is still an evidence case. The question is whether the State can prove intoxication beyond a reasonable doubt, and whether the investigation fairly ruled out other explanations.

Why these cases happen

DWI investigations rely heavily on an officer’s perception, especially early in the stop. Officers are trained to look for cues and to use standardized field sobriety tests (SFSTs). The problem is that SFST performance can be affected by many factors that are not alcohol or drug intoxication.

Even NHTSA materials recognize the need to assess possible medical impairment during SFST administration, including specific checks before the Horizontal Gaze Nystagmus (HGN) test such as equal pupil size and equal tracking. NHTSA also acknowledges that drugs other than alcohol can produce HGN.

When an investigation starts with assumptions, people with legitimate medical issues can get treated like they are intoxicated.

Medication and side effects that can be mistaken for intoxication

A significant number of prescriptions and over the counter medications can cause drowsiness, dizziness, delayed reaction time, or coordination problems. That does not mean medication automatically explains a DWI arrest, and it does not mean you should stop prescribed treatment. It does mean that a defense must examine whether the officer correctly interpreted what they saw and whether they considered alternatives.

In the defense context, medication-related cases often turn on:

• What the driver actually took, when they took it, and at what dosage

• Whether the medication has known side effects that match the observed behavior

• Whether there were interactions between multiple medications

• Whether the officer documented questions about medications, health, and symptoms

• Whether video shows the driver functioning better than the report suggests

NHTSA SFST guidance includes steps specifically designed to screen for medical impairment factors during the process. If the officer skipped those steps, rushed the testing, or misapplied the procedures, it can matter.

Fatigue and sleep deprivation can look like impairment

Fatigue creates real driving risk. NHTSA warns about the dangers of drowsy driving and provides education on recognizing fatigue and avoiding driving while sleepy.

From a defense standpoint, the point is not that fatigue is “fine.” The point is that fatigue can mimic the same cues officers associate with intoxication: slow responses, difficulty following instructions, poor coordination, and lane issues. Many people arrested on suspicion of DWI were actually coming off a night shift, dealing with insomnia, traveling long distances, or operating on very little sleep.

When fatigue is a legitimate factor, the defense looks closely at:

• Work schedule and sleep history in the 24 to 48 hours before the stop

• Whether the officer noted yawning, heavy eyelids, or other fatigue indicators

• Whether the stop occurred at a high-risk time window for drowsy driving

• Whether the officer’s narrative matches what the video actually shows

Medical conditions that commonly cause confusion

I am not providing medical advice here. The point is that some medical issues can create symptoms that resemble intoxication, and those issues should be taken seriously in a DWI defense strategy.

One example is hypoglycemia. Mayo Clinic notes that severe diabetic hypoglycemia symptoms can include confusion, clumsiness or loss of coordination, and trouble speaking or slurred speech. Those are exactly the kinds of cues officers often write down as impairment.

Other categories of conditions that can create similar “looks impaired” observations include:

• Vestibular or inner ear disorders that affect balance

• Neurological issues that affect speech or coordination

• Acute stress responses and panic symptoms that affect cognition and physical control

• Vision problems that influence eye testing and instruction compliance

In these cases, the defense is built around corroboration: records, timelines, and objective proof.

Why SFSTs can be vulnerable in medical and fatigue cases

SFSTs are treated as standardized, but in the real world they are often administered roadside in poor lighting, uneven surfaces, traffic noise, and stressful conditions. Medical factors, fatigue, and even anxiety can affect performance.

HGN is a frequent point of confusion. NHTSA SFST materials instruct officers to perform pre-checks like equal pupil size and equal tracking and to recognize when medical disorders or injuries may be present. NHTSA also acknowledges that drugs other than alcohol can cause HGN.

A strong defense often involves comparing what the officer claims to have done to what the video shows actually happened.

How these cases are defended in practice

“Medical impairment” defenses are not built by arguing with the police report. They are built by developing a clean, provable narrative and testing the State’s evidence.

1) Build a timeline that explains what the officer observed

• Sleep history, meals, stress, and physical symptoms

• Medications taken, when taken, and why

• Where you were coming from and where you were going

• When symptoms began and whether they were documented

2) Gather objective evidence quickly

• Dash cam and body cam video

• Dispatch logs and timestamps

• Hospital or urgent care records if there was medical evaluation

• Relevant medical documentation when appropriate

3) Test whether the officer followed proper procedures

• Did the officer document questions about health, injury, or medications

• Were SFST instructions given correctly and in a standardized way

• Did the officer conduct the medical impairment screening steps during HGN testing

• Does the video support the stated “clues,” or contradict them

4) Evaluate the chemical testing path

If the case involves a breath or blood test, that evidence has to be analyzed on its own terms. A blood test does not automatically prove impairment without context, and the chain of custody and testing process matter. The legal definition of intoxication also covers impairment by substances other than alcohol, which is why the actual proof must be tested carefully.

5) Use expert review when warranted

In certain cases, it makes sense to consult a medical expert or toxicology expert, and in some cases an SFST expert, particularly when the officer’s methods deviate from NHTSA guidance.

What you should do if this happened to you

• Do not try to “explain it away” to law enforcement after the arrest.

• Write down what you remember about your sleep, medications, symptoms, and timeline while it is fresh.

• Preserve anything that helps document the situation, including messages, work schedule, and any medical paperwork.

• Get counsel early so video can be requested and preserved before it is overwritten.

Talk with a McKinney DWI defense attorney

If you were arrested for DWI in McKinney, Collin County, or Dallas County, and you believe medication, fatigue, or a medical condition played a role in how you appeared roadside, the defense has to be built on evidence and procedure.

Call my office for a confidential consultation, or send a message through the website form.

Citations:

https://statutes.capitol.texas.gov/GetStatute.aspx?Code=PE&Value=49.01

https://www.nhtsa.gov/sites/nhtsa.gov/files/documents/sfst_pm_refresher_manual.pdf

https://www.nhtsa.gov/risky-driving/drowsy-driving

https://www.mayoclinic.org/diseases-conditions/diabetic-hypoglycemia/symptoms-causes/syc-20371525