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The laws surrounding DUI when it comes to marijuana usage, vary, from state to state. Some states, have a zero tolerance policy. Frankly, that means if you have even a slight trace of it – in your blood (THC) – you are guilty by default. Some states, have a set THC limit. If the amount of THC in your bloodstream is higher than the limit (similar to alcohol), then you are guilty. In more generous states, the prosecution has to prove that the THC in your blood had a measurable impact on your abilities – and left you impaired.
Each state is different, when it comes to marijuana usage. Some of the more generous states – such as the ones who have a flexible policy, are more understanding – and have lighter penalties for usage of marijuana.