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PubMed is a free interface for searching MEDLINE, the most popular bibliographic database in the health and medical sciences. It contains references to millions of journal articles from biomedical journals and is updated daily.

Search operators explained

The words AND and OR are used in searching to combine search terms or search lines together.

OR retrieves records which contain any of your concepts. E.g. you might search for heart attack OR myocardial infarction  OR stemi to retrieve articles with any of those three terms. Use OR to combine alternative searches around the same concept.

AND retrieves articles which contain all your concepts. E.g. heart attack AND aspirin. Use AND to retrieve papers which relate two different concepts together.

Note: It is best practice to use UPPERCASE Boolean terms in PubMed: OR, AND, NOT

The NOT operator

The NOT operator can be used to exclude a search term or set of results from your search. 

For example if you are searching for articles about acquired immune deficiency syndrome, you may find that a search for aids finds articles about hearing aids.  Searching for aids NOT "hearing aids" will remove these articles from your search.

Use the NOT operator with caution as you may accidentally remove relevant records. 

The NOT operator can also be useful when comparing the results of individual search lines.

Advanced Search

The Advanced Search screen allows you to:

Combine searches from the Search History

Searches you perform are saved in the search history. To access your search history, click Advanced, located below the search box, and scroll down to History and Search Details:

Search history

Each search is given an ID number. This allows you to combine individual searches into a new search by using the Boolean operators AND, OR and NOT.

Click on the elipsis (...) under Actions of the search line you want to combine and select Add query: This adds your search line into the query box. In this example we have already searched for the MeSH terms for aspirin and myocardial infarction and they are in our search history. 

Combine search stage 1

Repeat the process for all searches you want to include, only this time select Add with AND, OR or NOT as required. We are using AND to combine our two MeSH terms. 

Combine search stage 2

Both searches now appear in the query box . Click Search to combine into one search.

Combine search stage 3


Search using the Advanced Search Builder query box

To make sure your search is as comprehensive as possible and ensure you retrieve the best results it is advisable to use Advanced Search to combine your MeSH searches with searches by textword, i.e. words anywhere in the record such as in the title or abstract. 

You can run textword and MeSH searches directly into the Add terms to query box. For textword searches, ensure the Add terms to query box is set to All Fields. If you know the MeSH term you require you can set the query box to search for MeSH Terms and insert the name of the known MeSH term. You can check for MeSH terms by looking them up in the MeSH database.

An example of a search using both MeSH and textword terms is:

Search #1            heart attack (all terms)

Search #2            myocardial infarction (all terms)

Search #3            STEMI (all terms)

Search #4            myocardial infarction (MeSH terms)

Search #5            combine 1-4 with OR

Search #6            aspirin (all terms)

Search #7            aspirin (MeSH terms)

Search #8            combine 6-7 with OR

Search #9            combine 5 and 8 with AND