Just like each type of drug has its own effects, each type also takes a different amount of time to metabolize and exacts a different toll on the body. Even drugs of the same type, such as opiates like Percocet and Vicodin, can clear the body at very different speeds.
The amount of time it takes a drug to leave your body is based on the drug’s half-life, or how long it takes for the liver and kidneys to break down and filter half of the amount of the drug in your bloodstream. This means that if a drug’s half-life is one hour, after one hour you’d have half as much of the drug in your blood as you did when you first took it. After two hours, you’d have a quarter of the drug left, and after three hours, an eighth. For most medical purposes, a drug is considered to have cleared your system after five half-lives, when only about three percent of the drug is left. Each drug has its own half-life, ranging anywhere from seconds to days.
How Far Can a Drug Test Look Back?
If a drug is consumed orally, such as when drinking alcohol, swallowing a pill, or eating a marijuana edible, then it will pass through the liver. As the body’s toxin filter, the liver protects you from any number of dangerous chemicals that might be in the food and beverages you eat and drink. Enzymes such as cytochrome P450 and monoamine oxidase help break down drugs into harmless compounds for the kidneys to cleanse from the bloodstream. Sometimes, the drug you consume isn’t active itself, but instead metabolizes into an active compound once it passes through the liver for the first time.
The liver also removes some drugs directly from the bloodstream instead of metabolizing them, and excretes them in bile that exits the body through the digestive tract. Most drugs, however, exit the body through the kidneys, which remove toxins from the blood as urine. The kidneys need water in order to function, so they work less efficiently when you’re dehydrated. Finally, trace amounts of drugs can also leave the body through your saliva, sweat, and even the air you exhale – that’s how a Breathalyzer can detect alcohol in your breath.
Different types of drug tests can trace whether you’ve consumed drugs for different amounts of time. When you use a drug, it doesn’t just leave metabolic byproducts in your urine and blood; it also can leave traces in your fat, hair and fingernails.
- Breath tests can detect if someone has alcohol active in his or her system. The average person can metabolize about one drink per hour.
- Blood tests’ effective ranges will vary based on the half-life of the drug but are generally sensitive for about one to three days. Marijuana edibles will linger longer, for up to seven days.
- Saliva tests range depending on the drug, but most can detect drugs used in the last 12 hours to three days.
- Urine tests will also differ depending on the drug’s half-life but can detect most drugs for about two to four days. Drugs that can linger longer include marijuana and PCP.
- Hair tests generally reveal drug use between one week and 90 days after use.
Other factors that can affect how you metabolize drugs include:
- Size: The larger you are, the less a given dosage of a drug is likely to affect you.
- Percentage body fat: The more body fat you have, the less sensitive you may be to the effects of certain drugs.
- Gender: Women and men have different profiles of liver enzymes, affecting how they metabolize some drugs, especially alcohol.
- Age: Infants and elderly tend to have poorer-functioning liver and kidneys, making them take longer to process drugs from their systems.
- Genetics: People of different ethnic groups may have genetic resistances or vulnerabilities to how they metabolize certain drugs, especially alcohol.
- Diet: Many foods, such as grapefruit juice, and many dietary supplements can change levels of enzymes in the liver, affecting drug metabolism.
- Tolerance: If you use a drug repeatedly, you may require more of it to achieve the desired effect.
- Disease: Certain diseases that affect the liver, kidneys, heart or brain can affect how your body metabolizes drugs.
Alcohol metabolizes relatively quickly, with the user’s blood alcohol concentration (BAC) dropping by about 0.015, or roughly one drink’s worth of alcohol, every hour. According to the National Institute on Alcohol Abuse and Alcoholism, one standard drink in the United States contains about 14 grams of pure alcohol. That’s the amount of alcohol found in about 12 ounces of beer, 5 ounces of table wine, or 1.5 ounces of 80-proof liquor.
Be careful with mixed drinks. Although they might not taste strong, they can contain two to four drinks’ worth of alcohol, and take accordingly longer to process from your body.
Using strong alcohol often dehydrates the user as well as depleting the body’s supply of vitamin B1, which is necessary to maintain certain structures in the brain that are important for memory. It also disrupts sleep, so sleeping while drunk results in lower-quality rest and more sleep deprivation the following day. Sleep deprivation causes impaired mental functioning, including slower reflexes and the risk of falling asleep while driving.
Hangovers are dangerous in and of themselves. Alcohol also leaves the user impaired for driving and other delicate tasks for about 16 hours after it’s cleared the body. Long-term alcohol use can damage the liver and the brain.
The half-lives of various benzodiazepines are fairly long, with the drugs lingering in the user’s system for the better part of a day. This can impair driving and other tasks.
Half-lives of some common benzodiazepines include:
- Alprazolam (Xanax): 6 to 12 hours
- Clonazepam (Klonopin): 18 to 50 hours
- Diazepam (Valium): 20 to 100 hours
- Flunitrazepam (Rohypnol): 18 to 26 hours
- Lorazepam (Ativan): 10 to 20 hours
Since many benzodiazepines take more than a day to clear the body, using them daily can cause concentrations of the drug to build up in the fat and blood. Because of this, benzodiazepines should not be used for more than two to four consecutive weeks.
Long-term effects of prolonged benzodiazepine use can include:
- Sedation or hangover
- Confusion and impaired thinking
- Memory problems
- Difficulty paying attention
- Loss of coordination
- Depression or emotional blunting
- Tolerance and dependence
Compared to many drugs, cocaine is quite quick to enter and exit the bloodstream. According to the National Highway Traffic Safety Administration (NHTSA), it can reach peak levels in the blood within 30 to 60 minutes of administration. From there, it has a half-life of about 50 minutes. The degree of actual intoxication it causes will vary from person to person.
After using cocaine, the user might experience a crash that can last from nine hours to four days. The crash can include irritability, depressed mood, fatigue, sleepiness or insomnia, and craving for more cocaine. Long-term use can rewire the brain’s reward system, making the user vulnerable to addiction.
Heroin and Prescription Opiates
According to the NHTSA, heroin has a very short half-life of only two to six minutes, after which it is metabolized into morphine. Codeine also metabolizes into morphine, while hydrocodone (Vicodin, Lortab) metabolizes into hydromorphone (Dilaudid), and oxycodone (Percocet) metabolizes into oxymorphone (Opana). Half-lives of these drugs include:
- Morphine: 1.5 to 7 hours
- Hydromorphone: 2.3 hours
- Oxymorphone: 9 to 11 hours
- Fentanyl: 3.7 hours
- Methadone: 10 to 60 hours
Long-term use of opiates damages the body’s endorphin system, which controls how you experience pain. When combined with the tolerance and dependence that often occur with regular opiate use, this paves the pathway to addiction.
The main active chemical in marijuana is tetrahydrocannabinol (THC). When smoked, THC levels in the blood peak within eight minutes, and when consumed orally, they peak within one to two hours.
THC’s half-life for infrequent users is about 1.3 days. Because THC can dissolve in fat, it will soak into the body’s fat stores and then slowly release over time back into the blood, prolonging its effects. This means that regular users of marijuana can experience a half-life of five to 13 days.
Long-term use of marijuana may cause impairments in memory, attention and information processing which can last for months or years after the user stops using. It can also build up a tolerance to the drug, which can lead to dependence or addiction. Those who smoke their cannabis will also experience the same damage to their lungs as smokers of other substances, as smoke contains tar, ash and carcinogens.
Prescription Amphetamine-Based Stimulants
Although amphetamine-based drugs can improve some types of performance, according to NHTSA, they can also cause inattention and promote high-risk-taking behaviors, such as speeding and rapid lane-changing, making them dangerous to consume while driving.
They have half-lives of several hours, including:
- Amphetamine (Adderall, Dexedrine): 10 to 12 hours
- Methamphetamine (Desoxyn): 6.4 to 15 hours
- Methylphenidate (Ritalin, Concerta): 2 to 3 hours
Prescription amphetamines can be safe for some people to use as instructed by a medical professional, but using them outside of a doctor’s recommendation can lead to tolerance, dependence and abuse. Long-term abuse of amphetamines can damage the brain’s dopamine system. This can cause schizophrenia-like psychosis, characterized by paranoid delusions, erratic or violent behavior, and visual and auditory hallucinations. If the abuse goes on long enough, the damage can also include symptoms similar to Parkinson’s disease from the dopamine depletion.
 Craig, D. (June 2010). “Oxymorphone Extended-Release Tablets (Opana ER) For the Management of Chronic Pain.” Pharmacy and Therapeutics. Accessed August 7, 2014.
 “Fentanyl 50 micrograms/ml Solution for Injection/Infusion.” (December 6, 2012). Amdipharm Mercury Company Limited. Accessed August 7, 2014.
 Sharma, P., Murthy, P., and Srinivas Bharath, M. M. (2012). “Chemistry, Metabolism, and Toxicology of Cannabis: Clinical Implications.” Iranian Journal of Psychiatry. Accessed August 7, 2014.
 Ashton, H. (2001). “Pharmacology and effects of cannabis: a brief review.” The British Journal of Psychiatry. Accessed August 7, 2014.
 Kimko, H.C., Cross, J.T., and Abernethy, D.R. (December 1999). “Pharmacokinetics and clinical effectiveness of methylphenidate.” Clinical Pharmacology. Accessed August 7, 2014.