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Andy Dick has gotten lucky if he mainly uses crack cocaine to get high, our law enforcement sources say ... because fentanyl isn't generally mixed in with the drug.

Two current federal Drug Enforcement Administration officials tell TMZ ... mixing fentanyl with crack's a fool's errand. The substance burns off with heat -- so putting it in the drug wouldn't produce the desired effect. Getty

The chemical structure of fentanyl breaks down once someone lights up the rocks ... and it loses its potency. We're told the drug will still have some effect, but it simply won't be as strong -- and it certainly won't have a lethal effect on a person.

Fentanyl is mostly snorted or injected ... making drugs like cocaine or heroin more likely to contain some of the killer powder. Despite his

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