The phrase "dummy dusts" refers to substances that often resemble illegal drugs, but are actually harmless. Some synonyms for "dummy dusts" include "fake drugs," "imitation substances," and "placebo powders." Other terms that can be used include "mock drugs," "bogus powders," and "substitute substances." These phrases are often used in the context of movies and TV shows where actors need to simulate drug use without actually consuming illegal substances. While not harmful, these "dummy dusts" can still pose a risk if they are mistaken for real drugs and consumed. It is important to use caution when handling any substance that appears to be drug-related.