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This depends on many factors. It will only show up positive as long as there is some of the drug left in the body. Some of the things that determine how long it is would be which drug specifically it is, what the person's height, weight, and metabolism are like, how much muscle mass vs fat the person has, how quickly the person's body rids itself of the drug.
Some examples of times that we generally use are as follows:
Alcohol - 0-8 hrs
Marijuana starts showing up as soon as 6 hours, and can stay in a person who usually does NOT use it for up to 10 days, in a habitual user it can remain for months.
Cocaine can last up to 4 daysHere is a link
to a website that has a tremendous amount of information about drug testing. It also lists many other drugs of abuse and how long they remain in the body.
If you have any follow-up questions please feel free to ask them.
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