how long after smoking weed can i drive

Can you drive after smoking cannabis?

Table of Contents

What is cannabis?

Cannabis, otherwise known as Marijuana, comes from the dried parts of the cannabis plant; this can include the flowers, seeds, leaves, and stems.

It is known by all sorts of names, for example, pot, weed and hash. While many people choose to smoke or vape it, you can also consume cannabis in food, brewed tea, or oils.

There are two different parts of cannabis, CBD (cannabidiol) and THC (tetrahydrocannabinol). CBD is known for the effects that come along with pain relief and relaxation; it is common to find oils, creams and skincare products with CBD extract in. It is legal to vape CBD in the UK.

THC is most often associated with the adverse psychological effects of cannabis; products containing THC are illegal in the UK.

There are different methods of taking the drug, and each method may affect your body differently. If you inhale marijuana smoke into your lungs, the drug is then quickly released into your bloodstream and goes to your brain and other organs. It can take a little longer to feel the effects if you choose to eat or drink it. There is ongoing controversy around the effects of Marijuana on the body. People report various physical and psychological effects, from the feeling of harm, anxiety and discomfort to pain relief, calm and relaxed.

Some of the symptoms and long-term effects that can occur as a result of using cannabis are:

  • Impaired judgement.
  • Memory problems.
  • Increased or decreased symptoms of depression.
  • Increased release of dopamine.
  • Burning mouth.
  • Lung irritation.
  • Weakened immune system.
  • Phlegmy cough.
  • Bronchitis.

Is cannabis legal in the UK?

The United Kingdom approved an application for the use of medical cannabis at the end of 2018. Numerous studies have shown that AIDS, several types of cancer, glaucoma and epilepsy can be treated with pain relief based on medical marijuana. Even though it may seem different, medical marijuana is the same as regular marijuana, except that it requires a prescription from a professional doctor.

What are the rules around cannabis and driving?

If you do choose to consume cannabis, you may believe that it is okay to drive. Either straight after you have consumed the cannabis or within at least a few minutes of consuming it. In the UK, driving with alcohol in your system has an upper limit. You are allowed a small amount of alcohol in your system and are still able to get behind the wheel of a car with no issues. Whereas when it comes to cannabis, the UK operates a zero-tolerance policy of driving with drugs in your system.

What is the legal limit of cannabis allowed?

The legal limit of THC allowed in your bloodstream when getting behind the wheel of a car is 2mg (micrograms). If you were to smoke an average-sized joint, it usually contains around 120mg of THC. Realistically, it is possible to have only one single puff of cannabis, and you could quickly be over the limit for driving already.

The half-life of drugs is the amount of time it takes for that starting amount of drugs in your system to reduce by half. It is difficult to work out the half-life of THC metabolites as not every joint will have the same amount of cannabis in; THC contains more than one hundred metabolites.

These all have differing half-life rates, and the time it takes to reduce by half depends on certain things like your height and weight, your metabolic rate and if you take any medication.

How long should you wait before driving?

It is all dependent on how much cannabis you have consumed. If you take an average joint, which generally contains 120mg of THC, it should take around 12 hours before your blood level drops to the 2mg limit.

It is advised that you should not drive much sooner than 24 hours after consuming cannabis. It is highly likely that after 24 hours, the majority of the THC in your system will be gone but this cannot be guaranteed.

What is the legal penalty for being caught driving under the influence of cannabis?

Cannabis is still a drug that is illegal in many countries, and the UK is one of those countries. If you were to be caught under the influence of drugs by the police, you could face a minimum of a twelve-month driving disqualification with the chance of up to six months in prison. Driving under the influence of a drug is an offence that is taken very seriously in the UK, and it would be best not to risk doing so.

If your roadside drug swab came back positive

The UK police currently use a device known as DrugWipes, to test for cocaine and cannabis in your bloodstream at the roadside. All DrugWipes come with instructions. The police may have made a mistake.

How to spot for a mistake?

  1. Did the officer touch the sampling pads when they removed the blue tester?
  2. Did the officer not hold the DrugWipe bottom down while waiting?
  3. Did the officer wait less than 8 minutes?
  4. Did the test take much longer than 8 minutes?
  5. Was the test not rested horizontally while analysing?
  6. Did the officer use his torch to examine the DrugWipe?
  7. Did a faint line appear?

A roadside DrugWipe cannot account for impairment. It only tests for traces of cannabis and cocaine in your bloodstream. It is possible to fail the test even if you are not impaired in any way.

Key Takeaway

If you have been arrested on suspicion of driving under the influence of drugs, it was likely because you failed the swab test. These tests are non-evidential, which means they cannot be used in court, and they do not show whether you exceed the allowed limit of Cannabis or Cocaine. You could fail the swab test but pass the blood test. Any person who fails the roadside test for cannabis will almost certainly fail the blood test too.

If you have found this article helpful and think you may need legal advice around driving under the influence of drugs, speak to one of our friendly drug driving solicitors today who will treat your query with empathy and compassion.

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Cannabis, otherwise known as Marijuana, comes from the dried parts of the cannabis plant; this can include the flowers, seeds, leaves…

Smoking weed: When is someone too high to drive?

It used to be the stuff of stoner comedies and “Just Say No” campaigns. Today, marijuana is becoming mainstream as voters across the country approve ballot questions for legalization or medical use.

In response, state governments are testing ways to ensure that the integration of this once-illicit substance into everyday life doesn’t create new public health risks. These efforts are sparking a difficult question: At what point is someone too high to get behind the wheel?

The answer is complicated. Brain scientists and pharmacologists don’t know how to measure if and to what extent marijuana causes impairment.

The reason: Existing blood and urine tests can detect marijuana use, but, because traces of the drug stay in the human body for a long time, those tests can’t specify whether the use occurred earlier that day or that month. They also don’t indicate the level at which a driver would be considered “under the influence.”

“It’s a really hard problem,” said Keith Humphreys, a psychiatry professor and drug policy expert at Stanford University in California, the first state to legalize medical marijuana and where recreational pot use among adults became legal in 2016. “We don’t really have good evidence — even if we know someone has been using — [to gauge] what their level of impairment is.”

Marijuana is now legal for recreational use in 10 states and the District of Columbia — including Michigan, where a ballot initiative passed in November took effect Dec. 6. In New York, the governor said Dec. 17 that legalization would be a top priority for 2019. And nearly three dozen states have cleared the use of medical cannabis.

For alcohol, there is a clear, national standard. If your blood alcohol content (BAC) is 0.08 percent or higher, you’re considered cognitively impaired at a level that is unsafe to drive. Extensive research supports this determination, and the clarity makes enforcement of drunken driving laws easier.


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Setting a marijuana-related impairment level is a much murkier proposition. But states that have legalized pot have to figure it out, experts said.

“You can’t legalize a substance and not have a coherent policy for controlling driving under the influence of that substance,” said Steven Davenport, an assistant policy researcher at the nonprofit Rand Corp., who specializes in marijuana research.

Marijuana, after all, weakens a driver’s ability to maintain focus, and it slows reflexes. But regulators are “playing catch-up,” suggested Thomas Marcotte, a psychiatry professor at the University of California-San Diego and one of a number of academics around the country who is researching driving while high.

States have put forth a bevy of approaches. At least five have what’s called a “per se” law, which outlaws driving if someone’s blood level of tetrahydrocannabinol, or THC, exceeds a set amount. THC is marijuana’s main intoxicant.

“You can’t legalize a substance and not have a coherent policy for controlling driving under the influence of that substance.”

Colorado, where voters approved legalization of recreational marijuana in 2012, has this type of driving law on the books. It took three years to pass amid fiery debate and deems “intoxicated” any driver who tests higher than 5 nanograms of THC per milliliter of blood.

Rhode Island, Pennsylvania and Indiana are among states that forbid driving at any THC level. Still others say drivers should be penalized only if they are impaired by the chemical — a standard that sounds reasonable but quickly gets difficult to measure or even define.

None of these approaches offers an ideal solution, experts said.

“We’re still definitely evaluating which policies are the most effective,” said Ann Kitch, who tracks the marijuana and driving issue for the National Conference of State Legislatures.


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States that set a THC-level standard confront weak technology and limited science. THC testing is imprecise at best, since the chemical can stay in someone’s bloodstream for weeks after it was ingested. Someone could legally smoke a joint and still have THC appear in blood or urine samples long after the high passes.

There’s general agreement that driving while high is bad, but there’s no linear relationship between THC levels and degree of impairment. States that have picked a number to reflect when THC in the bloodstream becomes a hazard have “made it up,” argued Humphreys.

“The ones who wrote [a number] into legislation felt they had to say something,” he said. But “we don’t know what would be the analogy. Is the legal amount [of THC] equal to a beer? Is that how impaired you are? Is it a six-pack?”

Roadside testing for THC is also logistically difficult.

Blood, for instance, needs to be analyzed in a lab, and collecting urine gets complicated.

In Canada, which legalized recreational pot just this year, law enforcement will test drivers with a saliva test called the Dräger DrugTest 5000, but that isn’t perfect, either.

Some private companies are trying to develop a sort of breathalyzer for marijuana. But Jonathan Caulkins, a drug policy researcher at Carnegie Mellon University, said, “There are fundamental issues with the chemistry and pharmacokinetics. It’s really hard to have an objective, easy-to-administer roadside test.”


News MAP: See the states where marijuana is legal

Some states rely on law enforcement to assess whether someone’s driving appears impaired, and ascertain after the fact if marijuana was involved.

In California, every highway patrol member learns to administer “field sobriety tests” — undergoing an extra 16 hours of training to recognize the influence of different drugs, including marijuana. Because medical marijuana has been legal there since 1996, officers are “very used” to recognizing its influence, said Glenn Glazer, the state’s coordinator for its drug recognition expert training program.

That kind of training is taking off in other states, too, Kitch said. Lobbying groups such as Mothers Against Drunk Driving are pushing to bump up law enforcement training and rely on officers to assess whether a driver is impaired.

These tests, though, risk their own kind of error. “They are subjective,” Rand Corp.’s Davenport warned.

For one thing, officer-administered tests can be influenced by racial bias. Someone who has previously had poor experiences with law enforcement may also perform worse, not because of greater impairment but nervousness.

Indeed, relying on more subjective testing is in some ways the direct opposite of conventional wisdom.

“A general pattern of the last … 40 years is to try to take human judgment out of decision-making processes when possible. Because we fear exactly these issues,” Caulkins said. “The idea that you could come up with a completely objective test of performance … is ambitious.”

Researchers like Marcotte are trying to devise some kind of test that can, in fact, gauge whether someone is showing signs of marijuana impairment. But that could take years.

In the meantime, the public health threat is real. States with legalized pot do appear to experience more car crashes, though the relationship is muddled. “This is going to be a headache of an issue for a decade,” Caulkins said.

Kaiser Health News (KHN) is a national health policy news service. It is an editorially independent program of the Henry J. Kaiser Family Foundation which is not affiliated with Kaiser Permanente.

Now that more states are legalizing cannabis, many are tackling the question of how to define driving under the influence of weed. ]]>