California DMV Written Test: Alcohol, Drugs & Impairment Guide

Why this section trips up first-time test takers

The impairment questions on the California written knowledge test are not designed to be tricky — but they are designed to be thorough. California treats alcohol and drugs the same way when it comes to driving, which surprises a lot of people who assume a doctor's prescription gives them a free pass. It doesn't.

The good news: once you understand the handful of numbers and the logic behind them, these questions become some of the easiest on the test. Let's walk through each piece.

Effects on driving: it's about ability, not just legality

California's rule is simple to state: it is illegal to drive while under the influence of alcohol or any drug that affects your ability to drive safely. That includes illegal drugs, prescription medications, and over-the-counter products. The law does not draw a line between them.

Impairment shows up in ways the test loves to describe:

Slower reaction time to what you see and hear
Blurred or double vision
Poor judgment about speed and distance
Drowsiness or loss of focus
Trouble staying in your lane or holding a steady speed

Age matters here too. As you get older, your tolerance to alcohol drops, so the same drink can hit you harder than it used to. And cough syrup or other liquid medicines that contain alcohol count as alcohol.

How the test phrases it: "Taking any drug that affects your ability to drive — including prescriptions or over-the-counter medications — is illegal before driving." Notice the question usually asks whether a prescription is an excuse. The answer is always no.

Worked scenario: Dana takes an allergy pill that makes her drowsy, then drives. Even though the label says "non-drowsy," she feels foggy. If stopped, she can be cited for DUI because what matters is whether the drug affects her driving, not whether it came from a pharmacy.

BAC limits: four numbers to memorize

Blood alcohol concentration (BAC) is the measurement officers use. The test wants you to know which limit applies to which driver:

0.08% or higher — drivers 21 and older
0.01% or higher — drivers under 21
0.01% or higher — anyone on DUI probation
0.04% or higher — commercial driver's license holders and drivers carrying passengers for hire

One critical point the test loves: being under the limit does not mean you are safe to drive. Even a BAC below the legal threshold can impair you. The legal limit is a ceiling, not a safety guarantee.

How the test phrases it: "What is the legal BAC limit for a driver who is 21 or older?" The correct answer is 0.08%. A trap version asks about a commercial driver and expects 0.04%.

Worked scenario: Marcus is 22 and has two beers. He feels fine and estimates his BAC is around 0.05%. He is under the 0.08% limit, but if his driving shows impairment, he can still be charged. The number is not a shield.

Under-21 rules: zero tolerance in practice

Drivers under 21 face the strictest standard: 0.01% BAC or higher is illegal. That's essentially any detectable alcohol. If a driver under 21 is convicted, the consequences go beyond the criminal case — the DMV can suspend or revoke driving privileges and require a licensed DUI program. A separate rule applies to under-21 riders convicted of operating a bicycle while impaired.

How the test phrases it: "What is the legal BAC limit for a driver under 21?" Correct answer: 0.01%. Another version asks what happens if a minor's preliminary breath test shows alcohol — the answer involves license action and a DUI program, not just a warning.

Worked scenario: Priya is 19 and has half a glass of wine at a family dinner. Her BAC registers 0.02%. Because she is under 21, that is over her legal limit, and she faces license consequences even though an adult at that BAC would be under the 0.08% threshold.

Alcohol and cannabis in vehicles

California prohibits having an open container of alcohol in the passenger area of a vehicle while driving. The test also expects you to know that the state's DUI law covers drugs the same as alcohol — and that includes cannabis. Driving under the influence of cannabis is illegal, and you can be cited even if you use it legally.

A few vehicle rules worth knowing:

Open containers of alcohol belong in the trunk, not the cabin
Passengers cannot drink in the vehicle while you are driving
Having cannabis in the vehicle does not make it legal to drive impaired

How the test phrases it: "Is it legal to drive after using cannabis?" No — cannabis is a drug, and impairment applies. A related question asks where an open alcohol container should be kept. Answer: in the trunk.

Worked scenario: Sam has a closed bottle of wine in the back seat and an unopened six-pack on the floor. If an officer stops him, the open-container rules depend on whether the containers are open — but if Sam has used cannabis before driving, that alone can support a DUI charge.

DUI consequences: what happens after a stop

When you drive in California, you consent to a breath, blood, or urine test if an officer suspects DUI. If you refuse, the DMV can suspend or revoke your driving privilege for one year.

For a DUI conviction, expect license action and required completion of a licensed DUI program. If you are a first-time driver, the court can ask the DMV to delay your first license for up to a year. The details vary by case, but the test focuses on the big picture: license consequences plus a DUI program.

How the test phrases it: "What happens if you refuse a chemical test after a DUI arrest?" Correct answer: the DMV can suspend or revoke your license for one year.

Worked scenario: Alex is stopped on suspicion of DUI and refuses the breath test. Even before any court outcome, the DMV can act against his license for a year.

Quick self-check

1. What is the legal BAC limit for a driver who is 21 or older? A. 0.01% B. 0.04% C. 0.08%

2. A 19-year-old is stopped with a BAC of 0.02%. What is true? A. She is under 21, so 0.01% or higher is illegal for her B. She is under the adult limit, so no action applies C. Only commercial drivers face limits under 0.08%

3. Where should an open container of alcohol be kept while driving? A. In the front seat cup holder B. In the trunk C. On the passenger's lap

Answers: 1 — C. 2 — A. 3 — B.

Key takeaways

California treats alcohol, illegal drugs, prescriptions, and over-the-counter medications the same way: if they affect your driving, you cannot drive.
Memorize four numbers: 0.08% for 21+, 0.01% for under 21, 0.01% for DUI probation, and 0.04% for commercial and passenger-for-hire drivers.
Being under the limit does not mean you are safe or legal to drive.
Refusing a chemical test can cost you your license for a year through DMV action.
Open alcohol containers belong in the trunk, and cannabis counts as an impairing drug behind the wheel.

When you're ready to test yourself, try a few rounds of practice questions with Mia at lastdmv.com.

Frequently asked questions

Is a prescription medication an excuse for driving impaired?

No. California law does not distinguish between illegal drugs and medications from a doctor or pharmacy. If a drug affects your ability to drive safely, you can be charged with DUI.

What BAC is illegal for a driver under 21?

0.01% or higher. That is essentially any detectable alcohol, and a conviction can lead to license suspension or revocation plus a DUI program.

Can I drive after using cannabis if it's legal in California?

No. Cannabis is an impairing drug, and driving under its influence is illegal regardless of whether you use it legally.

What happens if I refuse a breath, blood, or urine test?

The DMV can suspend or revoke your driving privilege for one year.