How Field Sobriety Tests Handle Medical Conditions and Prescription Drugs

When a law enforcement officer stops a driver on suspicion of driving under the influence (DUI), roadside coordination tests are often among the first investigative steps. These exercises, developed and standardized with guidance from the National Highway Traffic Safety Administration (NHTSA), are intended to assess balance, coordination, eye movements, and the ability to divide attention between different tasks.

The problem is that these tests were developed largely around identifying alcohol-related impairment. They were not specifically designed to account for prescription medications, chronic health conditions, previous injuries, or other medical factors that can affect a person’s physical performance. As a result, symptoms caused by a legitimate medical condition may sometimes appear similar to what an officer expects to see in an impaired driver.

1. The Flawed Science of Field Sobriety Tests in Non-Alcohol Cases

Quick Answer: Standardized field sobriety tests were primarily validated in connection with alcohol impairment and blood alcohol concentration (BAC), rather than as reliable tools for determining impairment caused by prescription drugs or medical conditions.

The three primary standardized field sobriety tests—the Horizontal Gaze Nystagmus (HGN), Walk-and-Turn, and One-Leg Stand—were developed and standardized during the 1970s and 1980s. Their development focused heavily on identifying physical and behavioral indicators associated with alcohol consumption.

These tests do not establish a simple connection between poor performance and prescription-drug impairment or an underlying health condition. A driver dealing with joint pain, an inner-ear disorder, an old injury, or medication-related side effects may have difficulty completing the exercises even when they are not impaired.

For example, difficulty maintaining balance may result from an orthopedic problem rather than intoxication. Similarly, certain eye conditions can affect the observations made during an HGN examination. When an officer interprets every physical deviation as an impairment clue, legitimate medical explanations may be overlooked.

2. Common Medical Conditions That Mimic Impairment Clues

Quick Answer: A wide range of medical conditions—including neurological disorders, inner-ear problems, spinal injuries, diabetes, and certain eye conditions—can produce behaviors that resemble common impairment indicators.

During roadside testing, officers are trained to watch for particular physical responses. These may include swaying, stepping away from a designated line, using the arms for balance, difficulty following instructions, or unusual eye movements.

However, those same responses can occur because of medical problems that have nothing to do with alcohol or illegal drugs.

Stress can make these problems even more noticeable during a traffic stop. A person with an existing balance or mobility issue may perform substantially worse when asked to complete unfamiliar exercises beside a roadway, particularly while being observed by an officer.

That distinction can become important when challenging field sobriety evidence in a DUI case involving a medical condition.

3. How Prescription Medications Impact FST Performance

Quick Answer: Some prescription medications can influence balance, alertness, pupil size, muscle control, or reaction speed. These effects may affect field sobriety test performance without necessarily establishing that the person was incapable of driving safely.

Using medication exactly as prescribed does not automatically prevent law enforcement from investigating a driver for suspected impairment. At the same time, the presence of a prescription medication in someone’s system is not necessarily the same thing as proving actual driving impairment.

Different medications can produce different physical effects during roadside testing.

Sedatives & Anti-Anxiety Meds (Benzodiazepines): These medications can cause drowsiness, slower responses, or changes in coordination in some people.

Opioid Pain Relievers: Certain opioids can produce constricted pupils and sleepiness, observations that officers may associate with narcotic use.

Blood Pressure Medications: Some medications used to control blood pressure can contribute to dizziness or lightheadedness, particularly when changing position quickly.

Stimulants (ADHD Medications): Some stimulant medications may affect speech patterns, activity levels, pupil size, or general behavior, potentially creating observations that an officer associates with stimulant use.

A drug test can establish that a substance or its metabolites are present, but the presence of a substance alone does not necessarily answer the separate question of whether the driver was actually impaired at the time of the stop. This is one reason the circumstances surrounding the traffic stop, medical history, dosage, timing, and observed behavior can all become relevant.

4. The Role of Drug Recognition Experts (DREs) in Medical and Rx Stops

Quick Answer: Drug Recognition Experts (DREs) follow a structured 12-step evaluation when drug impairment is suspected, but their observations still require careful consideration when a driver has a medical condition or takes prescription medication.

If an officer obtains a 0.00% alcohol result but continues to suspect impairment, a Drug Recognition Expert (DRE) may be requested to conduct a more detailed evaluation.

DREs receive specialized law-enforcement training intended to help them recognize physical and behavioral indicators associated with different categories of drugs. Their evaluation can include observations of pupil size, eye movements, vital signs, muscle tone, coordination, and other factors. The process also includes questions concerning the person’s medical history.

However, specialized law-enforcement training is not the same as practicing medicine. Symptoms such as anxiety, tremors, unusual pupil responses, balance problems, or changes in muscle tone may have multiple possible causes.

For that reason, medical records, prescription information, physician testimony, and other objective evidence may be important when a DRE’s observations are being challenged.

5. Myths vs. Facts: Field Sobriety Testing for Non-Alcohol Cases

Myth: Failing a roadside balance test proves you were legally impaired by prescription medication.
Fact: Field sobriety exercises measure specific physical and cognitive behaviors under roadside conditions. Poor performance by itself does not establish the cause of that performance or prove therapeutic drug impairment.

Myth: You are legally required to perform physical field sobriety tests if you take prescription drugs.
Fact: The rules governing field sobriety testing differ by jurisdiction and circumstance. In many locations, some roadside exercises are voluntary, but drivers should understand their local laws before deciding whether to participate.

Myth: Having a valid doctor’s prescription protects you from a DUI arrest.
Fact: A valid prescription may demonstrate that the medication was lawfully obtained and used, but it does not necessarily prevent a DUI investigation or prosecution if authorities allege that the medication affected driving ability.

6. How to Protect Your Rights During a Traffic Stop

Quick Answer: If a medical condition or physical limitation could affect your performance, calmly explain that limitation to the officer. Before declining any roadside test, understand that the legal consequences of refusing can vary depending on the jurisdiction and type of test.

A traffic stop can become more complicated when a driver has a chronic condition, previous injury, or regularly uses prescription medication. Remaining calm and communicating clearly can help create an accurate record of the circumstances.

Be Polite and Cooperative: Provide your driver’s license, registration, and insurance information as required. Avoid unnecessary confrontation with the officer.

State Physical Limitations Early: If you are asked to perform balance or coordination exercises, explain relevant physical limitations, such as an injury, arthritis, vertigo, mobility problem, or other condition that could affect your performance.

Understand Test Voluntariness: Do not assume that every roadside test has the same legal requirements. Whether you can decline a particular test—and what consequences may follow—depends on the applicable state law and circumstances.

Seek Legal Representation: An experienced DUI lawyer in Salt Lake City can review the traffic stop, medical history, prescription records, field sobriety results, and available video evidence to determine whether medical factors provide a basis for challenging the prosecution’s case.

7. Challenging FST Results in Medical and Prescription DUI Defense

Challenging field sobriety evidence in a medical or prescription-drug DUI case generally requires showing that the driver’s performance had an alternative explanation unrelated to intoxication.

Several forms of evidence may help establish that connection:

Subpoenaing Medical Records: Medical records can document previous injuries, surgeries, arthritis, neurological conditions, spinal problems, or inner-ear disorders that could affect physical performance.

Expert Medical Testimony: Physicians, toxicologists, or other qualified experts may explain how a particular medical condition or therapeutic medication could affect balance, coordination, eye movements, alertness, or other observed behaviors.

Auditing Officer Testing Procedures: Dashcam and bodycam recordings can be reviewed to determine whether instructions were properly given, whether environmental factors affected performance, and whether the officer considered known medical limitations when interpreting the results.

Summary of Key Takeaways

Standardized field sobriety tests have important limitations when they are used to evaluate people with medical conditions or prescription medications. Although these exercises can provide officers with observations during a DUI investigation, poor performance does not necessarily identify the underlying cause.

A driver may struggle with balance, coordination, eye movements, or divided-attention exercises because of a medical condition, previous injury, medication side effect, or stress rather than alcohol or drug impairment.

Understanding these distinctions can help drivers and their attorneys evaluate roadside evidence more carefully. Medical records, prescription information, expert opinions, and video footage can provide objective context when physical test results are being used to support a DUI allegation.