Some health/medical styles require journal titles to be abbreviated but other styles you may need require them to be entered in full. If you enter abbreviations directly in to the Journal Title field in Endnote you would then not be able to switch to any styles which require journal titles in full.
Avoid entering articles twice by using Endnotes Term List feature. Endnote allows you to use term lists to apply quality control to author names, journal titles and keywords.
You can import medical, bioscience or chemistry journal term lists from the University of Queensland Library's site at http://www.library.uq.edu.au/faqs/endnote/journal_terms.html|
These term lists contain both abbreviations and full titles, thus allowing you to choose which you set for any particular style
Note if you are using Endnote on a networked PC you will need to copy styles into your H space and redirect Endnote to these files before making any further edits.