If PubMed finds a phrase it automatically searches the terms as a phrase rather than simply combining them. If you do not want this enter each term separated by the Boolean operator AND, e.g., asthma AND drug AND therapy .
Initials are optional.
Click on 'Limits' on the Features bar to go to the Limits page. Using pull-down lists you can restrict the search to a specific language, frequently used publication types such as review, and Entrez date, the date when a citation was added to the PubMed system. Use fill-in boxes if you want to restrict by publication date. Limits remain in effect until you remove the check to the left of 'Limits' on the Features bar. Terms inside a set of parentheses are processed as a unit and then incorporated into the overall strategy.