efficacious

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Bring to mind forgotten word acquaintances with
Merriam-Webster's Collegiate Thesaurus.
http://www.merriam-webster.com/cgi-bin/book.pl?thes.htm&2
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The Word of the Day for January 15 is: 

efficacious   \ef-uh-KAY-shus\   adjective
     : having the power to produce a desired effect

Example sentence:
     Doctor Copeland prescribed a new medicine that proved most 
efficacious in relieving Jonathan's nagging cough.

Did you know?
     "Effective," "efficient," and "effectual" are synonyms 
of "efficacious," but each of these words has a slightly 
different connotation. "Efficacious" suggests possession of a 
special quality or virtue that makes it possible to achieve a 
result ("a detergent that is efficacious in removing 
grease"). "Effective" stresses the power to produce or the 
actual production of a particular effect ("an effective 
rebuttal"), while "effectual" suggests the accomplishment of a 
desired result, especially as viewed after the fact ("measures 
taken to reduce underage drinking have proved 
effectual"). "Efficient" implies an acting or potential for 
acting that avoids loss or waste of energy ("an efficient small 
car").