How the Other Driver's Marijuana Use Can Impact Your Car Accident Case

The problem isn't proving you were injured by someone who used marijuana—the problem is proving that the marijuana use caused the accident and your injuries. Find out about some of the issues you might face.

By , J.D. · University of San Francisco School of Law
Updated by Dan Ray, Attorney · University of Missouri–Kansas City School of Law

While driving home from work one evening, you were hit by a driver who ran a stop sign. Police arrived on the scene and, after some investigation, concluded that the driver had recently used marijuana. In an insurance claim or a lawsuit to recover compensation for your injuries (what the law calls "damages"), will the other driver's marijuana use influence the outcome?

The answer is: Maybe, yes. If there's good evidence that the other driver's use of marijuana violated state law, that probably means your case is a bit easier to prove. But you'll still need to prove that the marijuana use caused your injuries. As we'll see, that can be a tall order.

The good news? Depending on the facts of your case, you might not need to rely on the other driver's drug use to prove your case. We'll explain why.

We begin with a brief overview of negligence (carelessness), the legal theory involved in nearly all car accident cases. We'll also explain how the other driver's marijuana use can make it easier for you to prove they were negligent. From there, we'll discuss some of the difficulties that can arise when you try to prove that another driver's marijuana use caused your injuries.

A Few Words of Caution

If you're here because you caused an accident while driving under the influence of marijuana, keep these points in mind.

Hire a Lawyer to Handle Your Pending or Potential Criminal Case

If you caused an accident while under the influence of marijuana and were cited or charged for it, your first call should be to a lawyer with experience handling marijuana-related criminal cases. Be sure to let your attorney know if anyone you injured or whose property you damaged notifies you they're bringing a claim for damages against you.

(Here's more about DUIs for drugged driving.)

Do you have auto insurance to cover the injuries and property damage you caused? If so, you're obligated to notify your insurance company of the wreck. If you don't provide timely notice, the insurer might use that as a reason to deny coverage for the accident.

(Learn more about the consequences of a DUI lawsuit for damages.)

Here too, let your lawyer do the talking for you. Don't speak to your (or the other driver's) insurance adjuster and don't give any statements—whether written, recorded or not—about what happened. Remember the line you hear all the time on television: Anything you say can be used against you. Things are bad enough. Don't make them worse by giving the prosecutor more ammunition.

Negligence and Negligence Per Se in a Car Accident Case

The overwhelming majority of car accident cases come down to proof that the responsible driver was negligent. Sometimes, it's possible to rely on a closely-related legal rule called negligence per se.


To prove an auto accident negligence claim, you must show that:

  • the other driver didn't drive as carefully as they should have under the circumstances, and
  • their lack of care caused your injuries.

Suppose, for example, that you're sitting at a red traffic light. The driver of the car approaching from behind is busy fiddling with the radio and isn't paying attention. That driver rear-ends your car and injures you.

These facts describe a classic case of auto accident negligence. The driver who rear-ended you didn't pay attention and wasn't as careful as they should have been. The law calls that negligence. If you can prove that their negligence caused you some injury, you can collect damages for the collision.

Negligence Per Se

In some car accident cases, you can skip the first step above—proving that the other driver wasn't as careful as they should have been. You do this by relying on a legal theory called negligence per se. Here's how it works.

When a driver violates a law that was designed to protect people injured in a car accident, the law presumes that driver's negligence. In other words, the violation of law is negligence per se—negligent in and of itself. Driving while under the influence of marijuana in violation of a state's driving under the influence (DUI) law is negligence per se.

What's the advantage of a negligence per se case? To win, you only need to prove what the law calls "causation"—that the other driver's violation of the DUI law caused your injuries. But as we'll see, in a driving under the influence of marijuana case (DUIM), proving causation can be a challenge.

Why Causation Can Be a Problem in DUIM Cases

Keep in mind what's required to prove causation in a DUIM case: You must show that driving while intoxicated by or under the influence of marijuana caused your injuries, at least in part. It's tempting to think that this must be easy. After all, look how simple it is in an alcohol intoxication case.

Field sobriety tests and breath and blood testing make proving alcohol impairment quick and easy. Ample research and studies document:

  • how and how quickly alcohol gets absorbed by the body
  • the effects of intoxication on consciousness, cognition, motor skills, and other physical and mental abilities involved in driving, and
  • how quickly alcohol gets eliminated from the body.

Unfortunately, the same isn't true for marijuana. A number of issues make it difficult to know how, how quickly, and for how long marijuana impairs physical and mental functions. Here's a partial list of those issues.

Different routes of administration. The main ingredient in marijuana that produces the "high" effect is called delta-9-tetrahydrocannabinol (THC). The two most popular ways to administer THC are by smoking and eating marijuana-containing products.

Inhaled THC produces intoxicating effects much more quickly than THC taken orally. Blood concentrations of THC vary significantly depending on the route of administration, as do the onset and duration of impairment. As a result, the time delay between using marijuana and testing for it can make a big difference in how much THC shows up in the blood.

Testing for marijuana isn't as easy as testing for alcohol. Few of the indicators of alcohol intoxication are present, or present to the same degree, with marijuana intoxication. Field testing for marijuana use and impairment can be particularly troublesome.

Because THC shows up in the breath only in minute quantities, and only for a short time after smoking, reliable breath testing isn't yet possible. Blood testing is possible but again, concentrations are directly correlated with the route of administration and a number of other factors.

The presence of THC doesn't necessarily mean it was taken recently. THC can remain in the body for days or weeks after marijuana use. How long it remains, and in what concentrations, will depend on factors like:

  • how much marijuana was taken
  • how it was taken
  • whether the user has other medical conditions that might impact how quickly THC gets eliminated, and
  • whether the user takes marijuana frequently or infrequently.

Weak correlation between positive test results and THC impairment. There's no doubt that THC produces physical and mental impairment. But in many cases it's extremely difficult to know when marijuana was taken based on its presence in the body.

For this reason (among others), positive testing doesn't necessarily mean the user is, or recently was, intoxicated or impaired due to marijuana use. Further, as noted by the Insurance Institute for Highway Safety, the amount of THC in the body doesn't reliably predict the level of a user's impairment.

All of these factors and more mean that proving causation—that your injuries were caused by the other driver's marijuana use—sometimes can be difficult. Obviously, some cases will be easier than others. If immediately after the wreck, the other driver rolls out of their car reeking of weed and showing visible signs of intoxication like a scene from a Cheech and Chong movie, that's reliable proof of impairment. Unfortunately, life rarely imitates art that closely.

Your lawyer can tell you more about your state's DUIM law and the impact it might have, if any, on proving causation in your case.

You Might Not Need to Rely on Marijuana Use to Win Your Case

What happens if you can't prove that a violation of your state's DUIM law caused your injuries? You still might have a case. Let's return to our opening example, involving a driver who ran a stop sign and hurt you. There are a couple of ways you can win your case.

Running the Stop Sign Can Be Negligence Per Se

In every state, running a stop sign is a violation of the motor vehicle code. Motor vehicle codes were designed to keep others safe from accident-related injuries. A violation of the motor vehicle code might be negligence per se. As long as you can prove that running the stop sign caused your injuries—which should be straightforward—you can collect damages.

Ordinary Negligence Is an Option, Too

If negligence per se doesn't apply in your case, you can still prove that running the stop sign was negligent. Driving with due care means obeying traffic signs and signals. A driver who doesn't stop at a stop sign is probably negligent. As long as you can prove that the negligence caused your injuries, you're entitled to damages.

Get Help With Your Car Accident Case

At first glance, it might seem that winning a case against a driver who hurt you while under the influence of weed should be easy. For the reasons we discussed, that's not necessarily so. In cases where THC intoxication isn't visibly apparent, proving a correlation between marijuana use, impairment, and the cause of your injuries can be a challenge.

Give serious thought to hiring an experienced car accident lawyer—one who's familiar with marijuana-related auto accidents—to handle your case. When you're ready to move forward, here's how to find an attorney near you.

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