How to Tell if Someone Is High: The Signs That Actually Mean Something
Red eyes, a sudden craving for chips, laughter that comes too easily. Most people can list the classic clues, and most of them are real effects of cannabis. Knowing how to tell if someone is high takes more than that list, though, because almost every sign also has an innocent explanation.
Parents worry about a teenager, hosts watch a guest who overdid it, and police officers have to decide at the roadside. The research shows which signs hold up, how long they last and where even trained officers get it wrong.
Key takeaways
- Red eyes, a faster heartbeat, dry mouth, a big appetite, memory lapses and anxiety are classic signs someone is high.
- No single sign proves cannabis use, since allergies, tiredness, anxiety and depression can look much the same.
- Smoking or vaping acts within minutes, while edibles can take 30 to 90 minutes to start and last 4 to 12 hours.
- In a 2023 study, officers using field sobriety tests labeled 49% of people who had smoked placebo cannabis as impaired.
- In a 2022 driving-simulator study, regular users felt their impairment wearing off before their driving actually improved.
- For a worried parent, a calm talk works better than an accusation or a surprise home drug test.
- If a child may have eaten an edible or symptoms look severe, call Poison Help at 1-800-222-1222 or 911.
Red eyes, a racing heart and other signs of being high
The signs someone is high come from THC acting on the body and the brain. Some of the short-term physical effects of cannabis use include increased heart rate, dry mouth, reddening of the eyes and the sudden appetite known as the munchies has been described for hundreds of years.
Psychiatrists use the same clues. Under the DSM-5, the American Psychiatric Association's diagnostic manual, cannabis intoxication means recent use, a marked change in behavior or mental state, and at least two of these signs within two hours, with no other cause to explain them:
- Red, bloodshot eyes
- A faster heartbeat
- Dry mouth
- A bigger appetite
The mental signs are harder to see but often more telling. Short-term memory suffers, so a person loses track of a story or asks the same question twice. Time stretches, and anxiety or paranoia can follow a strong dose. In a 2016 study of 122 cannabis users, planning, self-control, attention and coordination were all significantly worse after cannabis than after a placebo.
Potency and tolerance change how strong all of this looks. In our experience, two puffs that barely show on a daily smoker can leave a first-timer red-eyed and anxious, especially with a strain such as Cherry Poppers, a Lemon Kush and Cherry Z cross with THC levels reaching 31%.
Timing from the first puff to the comedown
Timing explains why someone can seem fine at first and very high an hour later. The American Academy of Pediatrics, or AAP, notes that smoking takes just seconds to minutes to act, but a THC edible typically takes 30 to 60 minutes after being eaten and digested. People who feel nothing at first may eat more, and the AAP links that delay to overdoses. Ranges vary with the source and the dose, and a 2021 emergency medicine review gives these:
| Stage | Smoked or vaped | Edible |
|---|---|---|
| First effects | Seconds to minutes | 30 to 90 minutes |
| Strongest effects | 15 to 30 minutes | 2 to 3 hours |
| Mostly worn off | Within 2 to 3 hours | After 4 to 12 hours |
| Stage | First effects | Strongest effects | Mostly worn off |
|---|---|---|---|
| Smoked or vaped | Seconds to minutes | 15 to 30 minutes | Within 2 to 3 hours |
| Edible | 30 to 90 minutes | 2 to 3 hours | After 4 to 12 hours |
A heavy dose stretches all of it, and a 2019 review notes that symptoms can persist up to 24 hours, depending on dose and tolerance.
Feeling lost on how to start? Try taking our quiz!
Start a QuizLimits of the classic signs
Every sign on the list has everyday look-alikes. The same 2021 review notes that no single body measurement or exam finding proves or rules out cannabis use, and even a fast heartbeat showed up in only 55% of the cannabis exposures reported to two poison control centers.
Each way of checking answers a different question:
| Method | What it can show | What it cannot show | Weak spot |
|---|---|---|---|
| Watching for signs | That something is off | Whether cannabis is the cause | Allergies, tiredness and depression look similar |
| Field sobriety tests | Poor balance and coordination | Whether THC is the cause | Labeled 49% of placebo users impaired in a 2023 study |
| Blood test | THC in the blood | How impaired the person is | Levels did not match driving skill in a 2022 study |
| Urine test or home kit | Use in the past days or weeks | Whether the person is high now | Home results need lab confirmation |
| Method | Watching for signs | Field sobriety tests | Blood test | Urine test or home kit |
|---|---|---|---|---|
| What it can show | That something is off | Poor balance and coordination | THC in the blood | Use in the past days or weeks |
| What it cannot show | Whether cannabis is the cause | Whether THC is the cause | How impaired the person is | Whether the person is high now |
| Weak spot | Allergies, tiredness and depression look similar | Labeled 49% of placebo users impaired in a 2023 study | Levels did not match driving skill in a 2022 study | Home results need lab confirmation |
Look-alikes for every sign
Anyone working out how to know if someone is high has to rule out the ordinary causes first. Allergies, a late night or a chlorinated pool can redden the eyes, and anxiety or coffee can speed up the heart. Hunger follows skipped meals and growth spurts, and a flat, distant mood can signal depression. Context tells you more than any single clue: the smell of cannabis, when the signs appeared, and whether several show up together.
Police tests and false alarms
Trained officers struggle too. In a 2023 University of California San Diego study, 184 adult cannabis users smoked either a THC joint or a placebo joint, and officers then ran field sobriety tests, the roadside checks of balance and coordination. One hour after smoking, they labeled 81% of the THC group and 49% of the placebo group as impaired, and officers suspected that 99% of those who failed the tests had received THC.
Regular users who seem sober
Tolerance adds another problem. A 2021 analysis of 128 cannabis users found only partial tolerance to feeling stoned among frequent users, and weak evidence of any tolerance to memory problems. In a 2022 UC San Diego driving-simulator study of 191 regular users, driving skills dropped most at 30 and 90 minutes after smoking, and blood THC levels showed no relationship with how well people drove. By the 90-minute mark, though, participants believed the impairment was wearing off and were more willing to drive. Feeling sober is a poor guide to being sober.
The unexamined day is a wasted opportunity. Reflect on what you did, what you learned, and how you can improve.
John Dewey
Teens, talks and home tests
Parents who type “Is my teen high?” into a search bar usually have a reason, such as red eyes at dinner or slipping grades. The AAP warns that some behaviors that suggest substance abuse, such as a flat affect and acting distant, could also be signs of depression. A flat affect means showing little emotion, and the AAP reminds parents that they could be wrong.
- Wait until everyone has cooled off and is sober before you bring it up.
- Describe what you noticed, such as red eyes or a missed curfew, without accusing.
- Ask open questions, like what your teen has heard about vaping at school.
- Offer a ride home at any hour, no questions asked.
- Bring changes in mood, sleep, appetite, grades or friendships to the pediatrician.
Home drug tests are tempting, but the AAP opposes involuntary testing and warns that home kits may give false or deceptive results. The FDA calls a positive home result preliminary until a lab confirms it, and even a confirmed result shows past use, not a current high, since tests cannot determine an approximate degree of impairment. Heavy users can test positive for 30 days or longer after they stop.
Helping a friend who got too high
Most people who get too high are not in danger, but they can feel awful, with a pounding heart, anxiety, paranoia or nausea. In our experience, a calm room and a friend who stays close do more than any trick:
- Move them somewhere quiet, with soft light and fewer people.
- Remind them that the feeling comes from cannabis and will pass.
- Offer water and a light snack, such as crackers or toast.
- Keep alcohol and more cannabis away, and take their car keys.
- Stay with them, and check that anyone who dozes off is breathing normally.
Some people also chew a few black peppercorns, one of several tricks for coming down from weed fast.
Most of the too-high nights we hear about start with a guest underestimating what they are smoking, so name the strain and its strength. Acapulco Gold, a Central American sativa at 26% THC, gives an energetic, long-lasting high, so a newcomer should take one puff and wait 15 to 30 minutes, the time inhaled THC takes to peak, before deciding on another.
When a high needs a doctor
Most bad highs end on their own, but some need a doctor, and young children are the biggest worry. In 2017, there were just over 200 reported cases of accidental consumption of cannabis edibles by children under six in US poison control data, and by 2021 the number had reached 3,054, a rise of 1,375%. Across the study, 22.7% of the children needed a hospital stay, and the vast majority found the edibles in their own home.
For a child or teen who has eaten an edible, the AAP's warning signs include panic, slurred speech, poor coordination, extreme sleepiness and breathing that stops for 10 seconds or longer. Call Poison Help at 1-800-222-1222, and if symptoms seem severe, call 911 or go to an emergency room right away.
Heavy, long-term use brings a different risk. Cannabinoid hyperemesis syndrome causes repeated bouts of vomiting and cramping belly pain in people who use cannabis weekly or more for months. Hot showers often ease it, but the only known cure is to stop using cannabis.
Signs as clues, not proof
Red eyes, a faster heartbeat, dry mouth, the munchies, memory lapses and anxiety are real signs of being high, but none of them proves cannabis use alone. Field sobriety tests flag sober people, regular users can feel fine while impaired, and drug tests show past use. Timing and context tell you more, a calm talk works better with a teenager than an accusation, and a child who may have eaten an edible needs Poison Help or 911 right away.
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