When a person has been stopped for the suspicion of driving under the influence, it is easier for the government to prove when the influence is of alcohol versus another drug such as marijuana. When the person driving is under the influence of alcohol, a simple breathalyzer will detect the amount of alcohol in the driver’s bloodstream. Alcohol generally remains in the system while the person is under its actual influence. In other words, when alcohol is in the person’s system, they under the influence of its effects. It does not remain in the system while the person is fully void of its influence. Therefore, a measure of the amount of alcohol in a person’s blood stream will indicate the effect on the person’s body.
Marijuana does not work in the same way. The effects of marijuana that render a person too impaired to drive only effect the person for moments after first smoking it. However, for weeks after, the substance remains in the person’s bloodstream.
Let’s consider an example. David smokes with his friends on Monday afternoon. He is under the influence for some thereafter, but is no longer under its influence Monday evening. David does not drive anywhere until the next day. On Friday he is pulled over by officers for suspicion of driving under the influence. David is not under the influence of anything, but if officer’s were to request he take a blood test, there would be evidence of marijuana in his blood stream. This would be the case even though David was no longer under the influence of marijuana.