In the phrase "appropriate substance use information", what noun does appropriate modify? I'm guessing it is information, is that correct?
Is this a general rule? How would you go about modifying any of the other 2 nouns?
Without context, this phrase is ambiguous. If you want the phrase to be less ambiguous without providing additional context, then you could reword the phrase to be more explicit.
Here are some examples:
Appropriate information about substance use
Information about using appropriate substances
Information on appropriately using substances
Without any context, I think most native speakers would assume that the meaning is "appropriate information". However, not everyone would make the same assumption, and the assumption may not be correct.