Police can tell if you’re high through a stacked process: what they see you doing behind the wheel, what they notice about your body and speech at the window, how you perform on roadside coordination tests, and — if the case goes further — a specialist’s medical-style evaluation and a lab test of your blood, urine, or saliva. There’s no breathalyzer equivalent for drugs, so detection leans heavily on trained observation, and the science behind each step is less settled than most drivers assume.
What Officers Notice Before You Say a Word
The assessment usually starts before the stop. Weaving between lanes, drifting off the road, braking for no reason, or driving well under the speed limit gives an officer the initial suspicion that something is off. None of these prove impairment on their own, but they create the legal basis to pull you over.
At the window, officers are trained to catalog physical signs quickly. Bloodshot or glassy eyes, pupils that look unusually large or small for the lighting, a flushed face, and visible tremors all go into the mental checklist. The smell of marijuana is one of the most commonly cited observations, and in many jurisdictions it has historically been enough to justify further investigation, though that’s shifting as more states legalize cannabis.
How you talk matters too. Slow or confused speech, difficulty understanding simple questions, fumbling with your license, or seeming disoriented all feed the officer’s decision about whether to move to roadside testing.
Roadside Field Sobriety Tests
If the officer suspects impairment, the next tool is a set of Standardized Field Sobriety Tests developed by the National Highway Traffic Safety Administration.1National Highway Traffic Safety Administration. DWI Detection and Standardized Field Sobriety Testing Participant Manual There are three:
- The Horizontal Gaze Nystagmus test, where the officer moves a pen or light and watches your eyes for involuntary jerking as they track to the edges.
- The Walk-and-Turn, which asks you to take nine heel-to-toe steps along a line, turn in a specific way, and walk back while the officer counts errors.
- The One-Leg Stand, which asks you to hold one foot about six inches off the ground and count aloud for 30 seconds.
Why These Tests Are Shakier for Drugs
Here’s what most drivers don’t know: these tests were designed and validated for alcohol, not drugs. The accuracy numbers you see quoted come from studies measuring whether officers could correctly flag subjects above a 0.08 blood alcohol level.2National Highway Traffic Safety Administration. Validation of the Standardized Field Sobriety Test Battery at BACs Below 0.10 Percent
For cannabis, the picture is much murkier. A 2023 clinical trial gave 184 cannabis users either THC or a placebo and then had officers evaluate them with field sobriety tests. Officers classified 81% of the THC group as impaired — but they also classified 49% of the placebo group as impaired. Nearly half the sober participants failed. The researchers concluded field sobriety tests are “useful adjuncts but do not provide strong objective evidence of THC-specific impairment.”3National Library of Medicine. Evaluation of Field Sobriety Tests for Identifying Drivers Under the Influence of Cannabis
The eye-tracking test is particularly weak for marijuana. Cannabis doesn’t reliably cause the involuntary eye jerking that alcohol and depressants produce, so the strongest test in the battery essentially drops out for the most commonly encountered drug.
A number of medical conditions can also cause someone to fail these tests without any substances in their system. Knee injuries, arthritis, and ankle problems make the Walk-and-Turn and One-Leg Stand difficult. Inner ear infections affect balance. Parkinson’s produces tremors. Diabetes can cause confusion and slurred speech when blood sugar swings. Even anxiety can make it hard to process rapid instructions under stress, which an officer may read as drug-induced confusion. NHTSA’s own materials acknowledge the tests are designed for “ideal conditions” that real roadsides often fall short of.4National Highway Traffic Safety Administration. SFST Administrator Guide
Drug Recognition Expert Evaluations
When alcohol doesn’t fully explain what the officer is seeing, the next step is often a Drug Recognition Expert. DREs are officers who’ve completed 72 hours of specialized classroom training plus supervised field evaluations before certification.5National Highway Traffic Safety Administration. Advanced Roadside Impaired Driving Enforcement Instructor Guide
They follow a standardized 12-step protocol developed by NHTSA and the International Association of Chiefs of Police, designed to answer three questions: are you impaired, is the impairment from drugs or a medical condition, and if drugs, which category.6International Association of Chiefs of Police. 12 Step Process
The evaluation includes a breath alcohol test to rule out alcohol as the sole cause, an interview with the arresting officer, a preliminary medical check with a pulse reading, more thorough eye examinations, divided-attention tests including a modified Romberg balance test and finger-to-nose, vital signs (blood pressure, temperature, second pulse), a dark room pupil measurement using a pupillometer, a muscle tone check, an injection site check with a third pulse, an interview with you, the DRE’s opinion about which drug category is responsible, and finally a toxicological sample sent to a lab to confirm or refute that opinion.7National Highway Traffic Safety Administration. Drug Evaluation and Classification Preliminary School Participant Manual
DREs classify impairment into seven categories: CNS depressants, CNS stimulants, hallucinogens, dissociative anesthetics, narcotic analgesics, inhalants, and cannabis. Each produces a distinct pattern — opioids constrict pupils and slow breathing, stimulants dilate pupils and raise vital signs, cannabis dilates pupils and raises pulse but produces no nystagmus.8National Highway Traffic Safety Administration. Drug Evaluation and Classification Preliminary School Participant Manual
DRE opinions are confirmed by toxicology about 84% of the time overall. Accuracy runs closer to 92% when only one drug is involved and around 80% when multiple substances are in play.9National Library of Medicine. An Analysis of Drug Recognition Expert Evaluations and Toxicological Outcomes
Blood, Urine, Breath, and Saliva Tests
After arrest, chemical tests are used to confirm what the officer or DRE observed. These lab results carry the most weight in court, but each has limitations when the substance isn’t alcohol.
Blood tests are the strongest for drug detection. They can identify a wide range of substances and their metabolites and provide the closest snapshot of what was in your system at the time of the stop. The detection window is short though, often just hours depending on the substance.
Urine tests detect a broader range of drugs over a longer period, from days to weeks. For chronic cannabis users, THC metabolites can show up for a month or more. That extended window is the problem: a positive urine test tells the court a substance was in your body recently, but says little about whether you were impaired at the time you were driving.
Breath tests are primarily useful for alcohol and don’t detect most other drugs, which is why blood or urine collection follows a drug-impairment arrest.
A newer option is roadside oral fluid testing — a saliva swab that screens for THC, cocaine, amphetamines, methamphetamine, benzodiazepines, and opiates within minutes. A few states, including Alabama and Indiana, have authorized statewide use, and several others have run pilot programs. Manufacturers themselves describe the results as presumptive, meaning they require laboratory confirmation before they’re reliable enough for court. Cross-reactivity with legal substances can produce false positives, and a positive result still doesn’t directly prove impairment.
The THC Problem
Cannabis creates challenges that don’t apply to most other substances. Active THC (delta-9-THC) is what causes impairment; inactive metabolites like THC-COOH are what your body produces after processing it. Standard urine tests detect the inactive metabolite, which can linger for weeks in regular users. Even blood tests can show residual THC in frequent users at levels above some states’ legal limits without any measurable impairment.
A major clinical trial found no relationship between THC blood concentrations and driving performance, with researchers concluding the results represent “strong evidence against the use of per se laws for cannabis.”10National Library of Medicine. Driving Under the Influence of Cannabis: Impact of Combining Toxicology Testing With Field Sobriety Tests Even so, about 18 states set specific THC thresholds for driving. Roughly half are zero-tolerance states, where any detectable amount of THC or its metabolite is treated as a violation regardless of impairment. A handful set a per se limit such as 5 nanograms per milliliter of blood.11Governors Highway Safety Association. Drug-Impaired Driving
Standard drug tests also can’t distinguish between delta-8 THC (legal under federal law and in many states) and delta-9 THC (still a controlled substance). Both break down into the same metabolite.
What You Can Decline and What You Can’t
This is where most drivers’ understanding breaks down. The line between roadside tests and post-arrest chemical tests is the key.
Field sobriety tests are voluntary in most states. You generally have the right to decline without facing automatic legal penalties like license suspension. Refusing doesn’t make the situation disappear. The officer can still arrest you based on their observations alone, and a prosecutor can bring up the refusal at trial. But you won’t be hit with the separate administrative penalties that come with refusing a chemical test.
Chemical tests after arrest work differently. Every state except one has implied consent laws, meaning that by driving on public roads you’ve already agreed to submit to chemical testing if lawfully arrested for impaired driving. Refusing typically triggers an automatic license suspension, often six months to a year for a first refusal, plus potential fines. In at least a dozen states, the refusal itself is a separate criminal offense.12National Highway Traffic Safety Administration. BAC Test Refusal Penalties
There’s one important constitutional limit. In Birchfield v. North Dakota, the U.S. Supreme Court held that while officers can require a breath test without a warrant after a lawful DUI arrest, they cannot require a blood test without either a warrant or your consent. States can impose civil penalties for refusing a blood test, but they cannot make refusal a crime on its own.13Justia US Supreme Court. Birchfield v. North Dakota, 579 U.S. (2016)
Since drug impairment usually requires a blood or urine test rather than a breath test, this ruling carries particular weight in drug DUI cases. Officers often need a warrant before drawing blood, though many jurisdictions have streamlined the process so a judge can approve one electronically in minutes.
Prescription Drugs Are Not a Free Pass
A valid prescription doesn’t shield you from a DUI. If a legally prescribed medication affects your ability to drive safely, you can be charged with impaired driving in every state. The law focuses on whether you were impaired behind the wheel, not whether you had permission to take the substance.
Some states recognize a limited defense for prescription use. To raise it, you’d generally need to show you had a valid prescription, took the medication as directed, and were not impaired beyond what your doctor’s instructions anticipated. Driving an hour after taking a sleep medication would undermine that defense regardless of how valid the prescription is.
Officers and DREs evaluate prescription impairment the same way they assess illegal drugs — same observations, same roadside tests, same 12-step evaluation. Toxicology will show the prescription drug in your system, and from there the question becomes whether the amount was consistent with prescribed use and whether you were actually impaired. If your medication carries a drowsiness warning on the label, a prosecutor can cite that label as evidence you knew the risk.