Use NUsearch, Northwestern University Library's search tool, to find books, documents, video, sound recordings, and other items across both the Evanston and Chicago campus libraries of Northwestern University.
NOTE: NUsearch can also search across several article databases to retrieve articles and other documents.                                  

opium AND trade--> finds both terms in the book or article records, or other search results.

opium AND (import* OR export*)--> useful for searching related or synonymous terms together.  finds opium and import, opium and export, or opium and both import and export.

* finds import, imports, importing... 

Also, create a list of possible related search words to try if needed.  For example: opium, opioid, opiate, heroin, morphine, opium poppies, etc.