PubMed

MeSH terms are the NLM's controlled vocabulary. A controlled vocabulary is a specified, pre-determined collection of terms (about 30,000 in this case). You cannot make up or add your own MeSH terms. Indexers (real live people!) read each article in PubMed/MEDLINE and tag them with a handful of MeSH terms. Then, when we search on that term, all articles tagged with that term are retrieved.

Why use a controlled vocabulary? The magic lies in fact that the number of terms is restricted, which helps overcome the fact that these databases do not have an intuitive search like Google does. In Google, you can search "used cars for sale" and you'll get "used SUVs for sale," "used Jeeps for sale," etc.  These databases are not like that - you get articles that correspond to the terms you searched on. Using a controlled vocabulary helps overcome that.  

Example:  Let's say you're interested in articles about pressure wounds. You'd also be interested in articles that use the phrases "pressure sores," "pressure injuries," and "pressure ulcers." Since the databases aren't intuitive, if you search on "pressure wounds" and the article only uses "pressure injuries" you won't get that article (you didn't tell the database you wanted articles about pressure injuries!). You would have to create a search string that contains all of the phrases above.

However - the controlled vocabulary would have only one term for this topic. In PubMed it's Pressure Injuries. The indexers who read an article about "pressure injuries" would tag that article with the Subject Heading "Pressure Injuries."  If the next article mentions only mentions "pressure wounds" it would also be tagged "Pressure Injuries."  If the next articles says "bedsores" it would also be tagged with "Pressure Injuries."

Then, by searching the subject heading "Pressure Injuries," you will get articles that use the words "pressure wounds," "pressure sores," "pressure injuries," and "pressure ulcers" - and even bedsores.

Searching with MeSH terms

MeSH clarifies

Suppose you wanted articles about how different cultures view a particular medical procedure.  If you search on the word Culture in PubMed without using MeSH, you get articles about tissue culture, Petri dish cultures, etc.  MeSH requires that you choose one (or more) subject heading related to culture, including ethnography, organizational culture, tissue culture techniques and more. 

MeSH broadens

Let's say you want to research injuries incurred by young pitchers.  These articles could be found by using the keywords "repetitive stress," "Little League Shoulder," "growth plate," "stress fracture" and many others. Instead of searching on each of those terms, you can use a single MeSH term (in this case, "repetitive stress") that captures them all.

However...

It is not always necessary to search for MeSH terms, as PubMed automatically searches for MeSH terms when possible.  If you search for a keyword for which a MeSH term exists, PubMed automatically searches for that MeSH term. Also, it can take up to six months for MeSH terms to be applied.  If you search using MeSH terms exclusively, you will miss the most recent articles.

To begin to search with the MeSH database, click on the PubMed logo (from any page)

This brings you back to the main PubMed search screen.  Look for the link for the MeSH Database (under "Explore")

This brings you to the MeSH database, where you can locate your MeSH terms.  To start, type a keyword into the search box. 

MeSH terms that pertain to that search term will appear.

Choose the most appropriate one(s) and they will auto-populate in the PubMed Search Builder to the right. 

You can click "Search PubMed" and you will be brought back to PubMed, and the results shown will be those citations returned by this MeSH seach.   Another way to search on MeSH terms is to COPY that phrase from the PubMed Search Builder. (shown above) then go back into PubMed and PASTE it into the PubMed search box. 

To get back to PubMed, change the drop-down box from "MeSH" to "PubMed."

Once you've got your MeSH term in the PubMed search box, you can add other keywords that pertain to the same topic behind it. Since MeSH terms are only applied to about 90% of articles in PubMed, it is always a good idea to search on the MeSH term as a keyword as well.