adjective
BAD : The medicine proved very efficient.
GOOD : The medicine proved very effective.
BAD : We need more efficient ways of dealing with pollution or the problem will get worse.
GOOD : We need more effective ways of dealing with pollution or the problem will get worse.
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efficient = working quickly and without waste: 'The more efficient the engine, the less petrol it uses.' 'Since the new software was installed, library services have become much more efficient.'
effective = having the desired effect: 'There are many effective ways of using computers for training purposes.' 'The advertisement was simple, but remarkably effective.'