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PubMed: Combining Terms

Boolean Logic: AND, OR, NOT

To add terms to your search you can use the Boolean operators AND, OR or NOT.

AND: Citations retrieved include all of the terms.

Using AND will retrieve fewer results because both terms must be present in the citation.


e-coli AND salmonella


OR: Citations retrieved include either term.

Using OR will retrieve more results because either term can be present in the citation.


anorexia OR bulimia


NOT: Will exclude a term from your search.

Use NOT with caution as it may exclude relevant results.


mutation NOT frameshift

Using Parentheses

Using parentheses changes the order in which PubMed processes the search statement. In order to build a more complex search, parentheses can be used around like terms that are combined with OR.


It's a good idea to get into the habit of using parentheses every time you combine search terms with OR so that you don't need to remember which way PubMed processes your search.