One of the reasons that drug tests are a colossal scam is that not all drugs stay in your system for the same amount of time. In fact, some of the least dangerous drugs such as marijuana can be detected in your urine, blood, and hair for much longer than harder drugs such as heroin and meth — meaning that schools, employers, and law enforcement agencies that drug test disproportionately penalize marijuana users.

In order to illustrate just how disparate drugs interactions can be, here's a breakdown of how long each drug stays in your system.

1. Alcohol

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Urine: Three to five days

Blood: 10 to 12 hours

Hair: Up to 90 days

2. Amphetamines

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Urine: One to three days

Blood: About 12 hours

Hair: Up to 90 days

3. Barbiturates

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Urine: Two to four days

Blood: One to two days

Hair: Up to 90 days

4. Cannabis

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Urine: Seven to 30 days

Blood: About two weeks

Hair: Up to 90 days

5. Cocaine

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Urine: Three to four days

Blood: One to two days

Hair: Up to 90 days

6. Heroin

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Urine: Three to four days

Blood: Up to 12 hours

Hair: Up to 90 days

7. LSD

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Urine: One to three days

Blood: Two to three hours

Hair: Up to three days

8. MDMA

Urine: Three to four days

Blood: One to two days

Hair: Up to 90 days

9. Methamphetamine

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Urine: Three to six days

Blood: 24 to 36 hours

Hair: Up to 90 days

10. Morphine

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Urine: Two to three days

Blood: Six to eight hours

Hair: Up to 90 days

With the exception of benzodiazepines (e.g. Valium), marijuana has the longest detectability. If you smoke a joint, it can be traced in your urine for seven to 30 days, depending upon your frequency of use and body type. That contrasts starkly with the detectability of more dangerous and addictive drugs, including heroin, meth, and cocaine.

To demonstrate why this contrasts matters, consider the fatal overdose rate.

More than 15,000 people died from drug overdoses involving heroin in 2017, the Centers for Disease Control and Prevention reported; heroin can only be detected in urine for three to four days. More than 16,000 people died from prescription painkiller overdose in 2017; potent painkillers such as morphine have a two to three day lifespan in urine.

Nobody has ever died from a marijuana overdose — but it can stay in your system longer than both of those drugs combined.