Speed (amphetamine) typically stays in your system for 1 to 3 days after a single use, though the exact window depends on the type of test and several personal factors. If you use it regularly, that window can stretch to a week or longer. Here’s what determines how quickly your body clears it and how long different tests can pick it up.
Detection Windows by Test Type
The most common drug test for amphetamine is a urine screen. After a single use, amphetamine shows up in urine for 1 to 3 days. Frequent or heavy use extends that window to roughly 5 to 7 days, because the drug accumulates in your system faster than your body can clear it.
Blood tests have a shorter detection window, generally 24 to 48 hours, since amphetamine concentrations in the bloodstream drop relatively quickly once the drug is metabolized. Saliva tests fall in a similar range, picking up use within the past 1 to 3 days. Oral fluid testing uses a lower threshold for a positive result (50 ng/mL for the initial screen compared to 500 ng/mL for urine), which can make it slightly more sensitive in the hours right after use.
Hair testing can detect amphetamine for up to 90 days, since traces of the drug get locked into hair as it grows. However, hair tests are considered less reliable than urine or blood tests and are used less often in standard screening.
How Your Body Processes Amphetamine
Once you take speed, your liver breaks down a portion of it while your kidneys filter the rest out through urine. According to FDA labeling data, the average elimination half-life of amphetamine is about 10 to 13 hours in adults, meaning it takes that long for your body to clear half the drug from your bloodstream. After five to six half-lives, the drug is essentially gone, which puts full elimination somewhere around 2 to 3 days for most people.
The split between liver processing and kidney excretion varies widely. Urinary recovery of amphetamine ranges from as little as 1% to as much as 75% of a dose, depending largely on urine pH. That enormous range means two people who take the same amount can clear it at very different rates.
Why Urine pH Matters So Much
Urine pH is one of the single biggest variables in how fast amphetamine leaves your body. Amphetamine is a weak base, and when your urine is acidic (pH around 4.5 to 5.5), more of the drug stays in its charged, water-soluble form inside the kidneys. That makes it harder for the drug to get reabsorbed back into your bloodstream, so you excrete it faster.
The difference is dramatic. Acidic urine can increase amphetamine excretion by up to 11-fold compared to alkaline urine (pH around 7.5 to 8.5). On the flip side, every one-unit increase in urinary pH above 7.5 can add roughly 7 hours to amphetamine’s half-life. So if your urine runs alkaline, whether from diet, supplements, or antacids, the drug lingers significantly longer.
Foods high in protein and cranberries tend to make urine more acidic, while diets heavy in fruits, vegetables, and dairy push it toward alkaline. This isn’t something you can precisely control in the short term, but it helps explain why detection times vary so much from person to person.
Other Factors That Affect Clearance
Beyond urine pH, several biological factors influence how long speed stays detectable:
- Liver function. Your liver relies on a specific enzyme family (CYP2D6) to break down amphetamine. People with reduced liver function, or those taking medications that compete for the same enzyme, will metabolize the drug more slowly.
- Kidney function. Since the kidneys handle a large share of excretion, any impairment in kidney function can extend the detection window.
- Age. The half-life of amphetamine is slightly shorter in children (9 to 11 hours) than in adults and adolescents (10 to 14 hours). Older adults with declining organ function generally clear drugs more slowly, though specific data on amphetamine in elderly populations is limited.
- Dose and frequency. A larger dose takes longer to eliminate simply because there’s more drug to process. Chronic use leads to accumulation in tissues, which is why regular users test positive for up to a week.
- Hydration and urine flow. Higher fluid intake increases urine flow rate, which can modestly speed up excretion. Dehydration does the opposite, concentrating the drug in a smaller volume of urine and slowing clearance.
What Counts as a Positive Test
Drug tests don’t just look for any trace of amphetamine. They use specific cutoff levels to separate a positive from a negative result. For a standard urine screen, the initial cutoff is 500 ng/mL. If the initial screen is positive, a more precise confirmatory test is run with a lower threshold of 250 ng/mL. Oral fluid tests use much lower cutoffs: 50 ng/mL for the initial screen and 25 ng/mL for confirmation.
These cutoffs matter because even after your body has mostly cleared the drug, tiny residual amounts may still be present but fall below the threshold. That’s why detection windows are estimates, not guarantees. Someone with alkaline urine and slow metabolism could test positive a day or two beyond the typical window, while someone with acidic urine might clear the same dose faster than expected.
Medications That Can Trigger a False Positive
Several common prescription and over-the-counter medications can cause a urine screen to incorrectly flag amphetamine use. The initial screening test works by detecting the general shape of the amphetamine molecule, and some unrelated drugs are similar enough to trigger a reaction. Known culprits include pseudoephedrine and phenylephrine (found in many cold and sinus medications), the antidepressants bupropion, trazodone, and fluoxetine, the diabetes drug metformin, the weight-loss medication phentermine, and certain antihistamines and decongestant nasal inhalers.
If you test positive and believe it’s a false result, the confirmatory test (which uses a different, more precise method) can distinguish true amphetamine from these cross-reactive substances. Letting the testing facility know what medications you take before the test can save time and prevent unnecessary complications.