In Michigan, we have two types of offenses for operating with drugs in your system.  First, we have an offense that applies to prescription drugs and marijuana card holders where the prosecutor must show that the substance impaired your ability to drive although the substance is legally in your system.  

Second, we have an offense where the mere presence of the drug is against the law; this applies to people without marijuana cards or prescriptions for the controlled substance.  

A substance like cocaine or heroin is not legal for anyone to have in their system, and it's certainly an uphill battle to defense this type of case, but it's not impossible.  When presence is enough to convict, the goal should be to discredit the test altogether.  The best way to do this is to challenge the process of of the blood draw, the transport and inventory of the sample, and the testing process.  If there's a reason to doubt the test results then there's reason to doubt the prosecutor's case, and the prosecutor can't convict without a valid test.  

While Michigan has adopted a general zero tolerance “presence” test for marijuana, this DOES NOT apply to medical marijuana cardholder under the Michigan Marijuana Act (MMMA). The Koon decision held that the MMMA trumps the Motor Vehicle Code’s provision which outlaws the operation of a car with active THC in one’s system.

The standard under Michigan law for a cardholder is “under the influence” similar to a drunk driving case involving alcohol. The only difference is there is no legal limit like the 0.08 BAC level for alcohol. For marijuana the test of this standard is a bit more vague. The burden of showing “under the influence” is of course on the prosecutor.
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A prosecutor will look to introduce the Defendant’s performance on field sobriety tests and the Drug Evaluation and Classification Program, which consists of 12 steps that evaluate the Defendant. The prosecutor may also rely on an expert witness to convey what if anything the levels indicate from the blood draw, and how the marijuana could put a driver under the influence. It is certainly more difficult for a prosecutor to proof “under the influence” for marijuana than alcohol.

If I polled 100 people in Michigan who smoke marijuana, I bet half of those people would not know that driving a car in Michigan is illegal with any amount of marijuana in their system. Many of them assume that they must be somehow “under the influence” of the drug, which impacts their ability to drive. Without a medical marijuana card in Michigan, the standard is “presence” meaning zero tolerance.

Many scientific studies have concluded that there is no link between THC levels and your ability to drive. In fact, many states acknowledge that, and have not adopted the zero tolerance standard like Michigan. In Michigan, any amount of active THC in your system would trigger a crime if you drive a car. It can be as little as 1 Nanogram.

Recent Michigan case law has held that inactive THC such as COOH-THC also known as 11-Nor-9-Carboxy-THC, which is the secondary metabolite of THC is NOT against the law to have it in your system while driving. While the law is still out of sync with the data on impairment, this ruling brought Michigan law closer to reality.

This means that you would have had to smoke marijuana within a few hours of driving to trigger a crime, because anything beyond a few hours should only leave the inactive metabolites. Without this ruling, one could technically smoke marijuana on May 1st, quit for 29 days and drive a car on May 30th, and still have inactive THC in their blood.