Field sobriety tests are often treated like hard science—but in reality, a lot of things can throw off the results, even if you haven’t had a drop to drink. Here are five big ones:

1. Medical Conditions and Physical Limitations

Inner ear problems, vertigo, old injuries, back or knee pain, neuropathy, obesity, or even simple balance issues can all make you “fail” balance-based tests like the Walk-and-Turn or One-Leg Stand. Many of these conditions are invisible and may never be noted properly by the officer.

2. Uneven or Unsafe Road Conditions

Field sobriety tests are often done on the side of the road—hardly a controlled environment. Things that can affect performance include:

• Sloped or uneven pavement
• Loose gravel, potholes, or debris
• High heels, boots, sandals, or slick shoes
• Rain, snow, ice, or strong wind

Any of these can cause a sober person to wobble, step off the line, or use their arms for balance.

3. Fatigue, Stress, and Anxiety

Being pulled over and told to perform tests while police are watching is stressful. Add in:

• Long work shifts
• Lack of sleep
• Adrenaline and fear

…and even a sober person can forget instructions, move too early, or appear unsteady purely from nerves and exhaustion.

4. Poor or Incomplete Instructions

Field sobriety tests are standardized, which means officers are supposed to give specific, detailed instructions and demonstrations. Results can be skewed if:

• Instructions are rushed
• The officer doesn’t demonstrate properly
• The person is interrupted or not allowed to ask questions
• There’s traffic noise, sirens, or language/accent barriers

If you don’t fully understand what you’re supposed to do, it’s easy to “fail” for reasons that have nothing to do with alcohol or drugs.

5. Medical or Natural Eye Issues (Especially for the HGN Test)

For the Horizontal Gaze Nystagmus (HGN) test (following a pen or light with your eyes), results can be affected by:

• Naturally occurring nystagmus (some people have it sober)
• Eye muscle fatigue
• Certain medications
• Head injuries or neurological conditions
• Improper distance, speed, or angle of the officer’s pen or light

If the test isn’t done correctly—or if the person has an underlying eye or neurological issue—the officer may see “signs of impairment” that aren’t caused by alcohol at all.

If you were arrested after “failing” field sobriety tests, that doesn’t automatically mean the tests were fair or accurate. A DUI defense attorney can look closely at how the tests were given, where they were done, and what was going on with you physically and medically to challenge the conclusions the officer drew.