magic bullet

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Something regarded as a magical solution or cure.

from the GNU version of the Collaborative International Dictionary of English.

• noun A drug or therapy or preventive that cures or prevents a disease, with only minimal side effects.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic A simple remedy to a difficult or complex problem, especially a cure for a disease. Often used to refer to a non-existent, overly simplistic remedy.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a remedy (drug or therapy or preventive) that cures or prevents a disease