Marijuana cases are where the gap between presence and impairment is widest. With alcohol, the blood level rises and falls in step with the effect, which is why a number works. THC does not behave that way. It spikes and then drops fast, its inactive metabolite lingers for days or weeks, and no blood level has been shown to mark impairment. A positive marijuana test is one of the weakest forms of proof the State can bring to a DUI.
Active THC in the blood spikes during use and then drops quickly, while the inactive metabolite stays detectable for days or weeks. A blood level drawn later tells you little about whether you were affected at the wheel.
Why a Blood THC Number Does Not Track Impairment
After smoking, THC peaks in the blood within minutes and then falls sharply as it distributes into tissue, often well before any blood is drawn. The effect a person feels does not move in lockstep with that curve. By the time a sample reaches the lab, the active THC may be low even if there was some effect earlier, or there may be measurable THC with no effect at all. Federal researchers have acknowledged that there is no blood THC concentration that reliably predicts impairment, which is the opposite of the situation with alcohol.
The government’s own report proves the number does not track the high
None of that is a defense theory. It comes straight from the 2017 NHTSA Marijuana-Impaired Driving Report to Congress. NHTSA told Congress that the level of THC in the blood and the degree of impairment are not closely related. THC peaks within minutes of smoking and then falls fast, dropping more than 80 percent within about half an hour, while the actual impairment does not peak until roughly 90 minutes later, long after the blood level has crashed. Because THC is stored in fat, it can still turn up in blood weeks after use, when no one is impaired at all. So the single number the State wants to wave in front of a jury, your blood THC level, moves in almost the opposite direction from the thing they are trying to prove.
| Time after smoking | Blood THC | Impairment |
|---|---|---|
| A few minutes | At its peak | Still coming on |
| About 30 minutes | Down more than 80 percent | Rising |
| About 90 minutes | Low | At its peak |
| Hours to days later | Can still be detectable | Gone |
A blood draw almost always happens hours after driving. By then the number and the impairment have parted ways, which is why NHTSA itself calls per se THC limits not evidence-based.
The Metabolite Problem
Most positive marijuana results lean on carboxy-THC, the inactive metabolite. It does nothing to your faculties, and it can stay detectable for days or weeks, especially in regular users. A result resting on carboxy-THC tells you a person used cannabis at some point, not that they were impaired behind the wheel. For eaten cannabis the picture shifts again, with delayed effects and a larger role for the active metabolite 11-hydroxy-THC, so a single number means even less.
How We Defend a Marijuana DUI
I start with what the lab measured and when the sample was taken relative to the driving. I separate active THC from the inactive metabolite, I weigh the timing against the known pharmacology, and I hold the State to the impairment theory, since Florida has no per se THC limit. Where the case also rests on the officer’s observations or a drug recognition evaluation, those become their own line of attack.
Medical Marijuana and the Regular User
Florida has a large medical marijuana program, and a lawful patient faces a particular bind. Holding a card does not excuse driving while impaired, so a prescription style defense does not end the case. The other side of that coin is stronger, because a daily medical user develops tolerance to the effects and carries measurable THC and carboxy-THC long after any effect has passed. A level that might suggest recent use in a first-time user can be a steady background reading in a patient who medicates every day, and the same logic reaches regular users generally. It is also worth being clear about what is not impairment. The smell of cannabis in a car, an admission that you used earlier in the day, or a coated tongue are things officers write down, and none of them measures whether your faculties were affected when you drove. The State still has to connect a real deficit to the driving, and with a tolerant regular user that connection is hard to make from a number alone.
A marijuana DUI is one of the most defensible cases I see, because the State’s own science undercuts its best exhibit. I take the blood number and show the jury what it really measures, which is that you used cannabis at some point, not that you were impaired when you drove. Florida law backs that up, since under West v. State, 553 So. 2d 254 (Fla. 4th DCA 1989), the mere trace of a drug is not proof of impairment, and there is no legal THC limit here at all. I know this chemistry, so I do not let a lab result stand in for the one thing the State has to prove.
I started out as an Assistant Public Defender in Florida’s Thirteenth Judicial Circuit, in Tampa, and today I am one of six ACS-CHAL Forensic Lawyer-Scientists in Florida. I work both the science and the procedure in your case the way the State’s own analysts and officers are trained to, and I show a jury the exact point where the evidence does not hold up. Learn more about my background.
Marijuana DUI Questions We Hear Most
Does a positive marijuana test prove I was impaired?
No. A positive test shows that THC or its metabolite was present, not that you were affected when you drove. There is no scientifically accepted blood THC level that marks impairment the way 0.08 does for alcohol, and blood THC falls quickly while any effect may rise, fall, or be absent on its own timeline.
Why does blood THC drop so fast?
After smoking, THC peaks in the blood quickly and then falls sharply within an hour or so as it moves into tissue, often before the blood is even drawn. A sample taken later can show a low level that says little about the moment of driving, which is one reason blood THC is a poor measure of impairment.
What about carboxy-THC on my report?
Carboxy-THC is the inactive metabolite. It cannot impair you, and it can stay detectable for days or weeks, especially in regular users. A result built on carboxy-THC is evidence of past use, not proof that you were affected at the wheel.
Are edibles different from smoking?
Yes. Eaten cannabis is absorbed and metabolized differently, the effect is delayed and prolonged, and an active metabolite called 11-hydroxy-THC plays a larger role. That changes how a blood level should be read and makes a simple number even less reliable as a measure of impairment.
Does Florida have a THC limit for driving?
No. Florida has not set a per se blood THC level. A marijuana case has to be proved under the impairment theory, where the State shows your normal faculties were affected, and the disconnect between blood THC and impairment is a serious problem for that proof.
Can a regular or medical user be charged?
Yes, but regular use cuts against the State as much as for it. A frequent user can carry measurable THC and carboxy-THC long after any effect is gone, and can be tolerant to doses that would affect a first-time user. Presence in that situation is especially weak evidence of impairment.
Does a high THC blood level prove I was impaired?
No. The 2017 NHTSA report to Congress found that blood THC level and impairment are not closely related, since THC peaks and falls within minutes while impairment peaks around 90 minutes later, and THC can linger for days or weeks after any effect is gone. Florida has no per se THC limit, and the State must prove your normal faculties were impaired.
Related pages: Presence is not impairment, parent drug vs metabolite, why there is no per se drug limit, and drugs and impairment in blood.
This page is general information, not legal advice, and it does not create an attorney-client relationship. Drug DUI in Florida is governed by Fla. Stat. 316.193, section 877.111, and chapter 893, and chemical testing is governed by Fla. Stat. 316.1932 and 316.1933. Procedures and rules change, and every case turns on its own facts. Past results do not guarantee a similar outcome.
