Testing for drug impairment is nothing like testing for alcohol impairment. With alcohol, the officer pulls out a breathalyzer, gets a number, and that number tells a fairly clear story. For drugs, though, there is no equivalent device. No roadside gadget can tell an officer how much of a substance is in your system or whether it is actually affecting your driving.
So how do police build a DWAI-Drugs case without that concrete number? Through a layered process that moves from initial observations at the roadside all the way through chemical testing at a station or hospital. Each layer adds a piece to the puzzle — and each layer has weaknesses that matter if you are the one facing charges.
Phase 1: The Initial Traffic Stop and Observations
Everything starts with the reason the officer pulled you over in the first place. Maybe you were swerving, driving unusually slowly, running a stop sign, or part of an accident. From the moment the officer approaches your window, they are documenting observations: your speech patterns, eye appearance, coordination getting out of the vehicle, whether there is an odor of marijuana or other substances, and your general demeanor.
These initial observations are what establish probable cause to investigate further. When the officer suspects impairment but is not sure whether alcohol is the cause, the investigation shifts toward drug-specific testing.
Phase 2: Field Sobriety Tests
The officer will likely ask you to perform standardized field sobriety tests (SFSTs) — the walk-and-turn, one-leg stand, and horizontal gaze nystagmus (HGN) test. These tests were originally designed to detect alcohol impairment, but officers use them as a starting point for drug cases, too.
One thing worth knowing: if you perform poorly on these tests but your breathalyzer comes back low or at zero, that mismatch is exactly what triggers the next phase. The officer sees signs of impairment that alcohol does not explain, which leads them to suspect drugs.
| Test | What It Measures | What Officers Look For |
|---|---|---|
| Walk-and-Turn | Divided attention, balance | Stepping off line, using arms for balance, wrong number of steps, inability to follow instructions |
| One-Leg Stand | Balance, time estimation | Swaying, hopping, putting foot down, using arms for balance |
| Horizontal Gaze Nystagmus | Involuntary eye movement | Lack of smooth pursuit, sustained nystagmus at maximum deviation, onset before 45 degrees |
These tests are voluntary. You can refuse them without triggering the automatic administrative penalties that come with refusing a chemical test. That said, your refusal may still influence the officer’s decision about whether to arrest you.
Phase 3: The Drug Recognition Expert Evaluation
This is the centerpiece of most drug impairment investigations. A Drug Recognition Expert (DRE) is a police officer who has completed additional training — certified through a program developed by the National Highway Traffic Safety Administration (NHTSA) and the International Association of Chiefs of Police — to identify drug impairment and determine which category of drug is causing it.
The DRE conducts a standardized 12-step evaluation that typically takes about an hour.
Steps 1–6: Building the Picture
Step 1: Breath Alcohol Test. The DRE reviews your breathalyzer results to determine whether alcohol alone explains the level of impairment the arresting officer observed. If your BAC is low but you appear impaired, the DRE proceeds with the drug evaluation.
Step 2: Interview of the Arresting Officer. Next, the DRE talks to the officer who pulled you over — what driving behavior they observed, how you acted during the stop, and whether you made any statements about drug use.
Step 3: Preliminary Examination and First Pulse. The DRE checks your general condition, asks about medical issues, looks at your pupils under normal lighting, and takes your pulse for the first time. This step also screens for medical conditions that might mimic drug impairment — the same conditions that can affect breathalyzer readings in alcohol cases.
Step 4: Eye Examinations. This goes beyond the basic HGN test from the roadside. The DRE checks for horizontal and vertical gaze nystagmus and tests your eyes’ ability to converge (cross). Importantly, different drug categories produce different eye movement patterns.
Step 5: Divided Attention Tests. Four psychophysical tests follow: the Modified Romberg Balance test (standing with head tilted back, eyes closed, estimating 30 seconds), the walk-and-turn, the one-leg stand, and the finger-to-nose test.
Step 6: Vital Signs and Second Pulse. Blood pressure, body temperature, and pulse are measured here. Different drug categories raise or lower these vital signs in predictable ways — stimulants tend to elevate everything, while depressants do the opposite.
Steps 7–12: Confirming the Drug Category
Step 7: Dark Room Examinations. The DRE measures your pupil size in three different lighting conditions: near-total darkness, indirect light, and direct light. Pupil size and reaction speed are among the most reliable indicators of specific drug categories. For example, cannabis typically dilates pupils, while opioids constrict them.
Step 8: Muscle Tone Check. The DRE examines your muscle tone by feeling major muscle groups. Some drug categories (like PCP and stimulants) cause rigidity, while others (like depressants) cause flaccidity.
Step 9: Check for Injection Sites and Third Pulse. The DRE examines your arms, neck, and hands for evidence of recent injections, then takes your pulse for the third time.
Step 10: Subject’s Statements and Interrogation. At this stage, the DRE asks you questions about drug use. Anything you say can and will be used against you, so your right to remain silent matters here just as much as at any other point.
Step 11: Opinion of the Evaluator. Based on the totality of the evaluation, the DRE forms an opinion about whether you are impaired and, if so, which of the seven drug categories is responsible: central nervous system depressants, CNS stimulants, hallucinogens, dissociative anesthetics, narcotic analgesics, inhalants, or cannabis.
Step 12: Toxicological Examination. Finally, a blood or urine sample is collected and sent to a laboratory for analysis. This test either confirms or fails to confirm the DRE’s opinion about which drug is present.
The Seven Drug Categories
DREs learn to classify impairment into seven categories. Each one produces a distinct pattern of physical signs that the 12-step evaluation targets.
| Drug Category | Common Examples | Key Physical Signs |
|---|---|---|
| CNS Depressants | Alcohol, benzodiazepines (Xanax, Valium), barbiturates, sleep aids | Slowed reflexes, slurred speech, uncoordinated, droopy eyelids |
| CNS Stimulants | Cocaine, methamphetamine, Adderall, Ritalin | Elevated vital signs, dilated pupils, hyperactivity, grinding teeth |
| Hallucinogens | LSD, psilocybin, MDMA | Dilated pupils, elevated temperature, disorientation, hallucinations |
| Dissociative Anesthetics | PCP, ketamine, DXM | Blank stare, rigid muscle tone, elevated blood pressure, nystagmus |
| Narcotic Analgesics | Heroin, oxycodone, fentanyl, morphine | Constricted pupils, drowsiness, slow breathing, low blood pressure |
| Inhalants | Paint thinner, nitrous oxide, aerosols | Disorientation, residue on face/hands, slurred speech |
| Cannabis | Marijuana, THC edibles, concentrates | Dilated pupils, bloodshot eyes, elevated pulse, impaired time perception |
Phase 4: Chemical Testing
After the DRE evaluation, police will request a blood or urine sample to confirm the presence of the suspected drug. This is where New York’s implied consent law comes into play — once police lawfully arrest you, refusing the chemical test triggers a mandatory one-year license revocation and a $500 civil penalty from the NY DMV, regardless of what happens with the criminal charge.
One critical distinction tends to get lost in most conversations about drug testing: a positive result only proves the drug was in your system. It does not prove impairment at the time you were driving.
This is especially relevant for marijuana. THC metabolites can remain detectable in blood for days and in urine for weeks after the impairing effects have completely worn off. As a result, a positive test taken hours after a traffic stop tells you almost nothing about whether the driver was actually impaired behind the wheel. For a detailed comparison of how DWAI-Drugs differs from alcohol DWI in terms of proof and defense strategy, we break it all down.
Where the Defense Opportunities Are
Every phase of this process has potential weak points, and an experienced DWI defense attorney will examine each one.
The Initial Stop
Was there a legitimate reason for the stop? If the officer lacked probable cause, then everything that follows — the FSTs, the DRE evaluation, the chemical test — could all face suppression.
The DRE’s Credentials and Protocol
Was the DRE properly certified and current on recertification requirements? DREs must maintain certification through ongoing evaluations and training. Beyond credentials, the protocol itself matters: did the DRE follow all 12 steps in order? Accurate recording of vital signs and pupil measurements matters too — as does adequate screening for medical conditions. Skipped steps or sloppy documentation can undermine the entire evaluation.
The Chemical Test
Did the lab properly collect, store, and test the sample? A proper chain of custody matters here. Were the lab’s accreditation standards met, and did the test occur within the required timeframe? These are not technicalities — they are the foundation of any credible result.
The DRE’s Opinion Itself
The DRE’s conclusion about which drug category caused impairment is an opinion, not a scientific measurement. A skilled defense attorney can challenge it by showing that the physical signs were consistent with fatigue, medical conditions, prescription medications taken as directed, or simply nervousness.
What You Should Know If You Are Stopped
If an officer pulls you over and suspects drug impairment, a few things are worth keeping in mind. First, field sobriety tests are voluntary — you can politely decline without automatic administrative penalties. The DRE evaluation and chemical testing happen after arrest, and refusing the chemical test carries its own separate consequences. You also have the right to remain silent during questioning, and exercising that right is almost always the right call. Finally, if you take any prescription medications, tell your attorney — not the officer — because that information shapes your defense strategy.
The DWI TEAM handles DWAI-Drugs and DWAI-Combination cases throughout New York. If you or someone you know went through a DRE evaluation and now faces drug-related driving charges, we can review the entire process and identify where the case can be challenged.
Disclaimer: This overview is for informational purposes only and does not constitute legal advice. Every case is unique — contact our New York DWI lawyers for personalized guidance.
