effective / efficient

If something is effective, it means it achieves the desired effect/result:

  • This vaccine is quite effective against the disease. or ( the vaccine has the desired result of preventing the disease)
  • The new law was ineffective in reducing crime. or ( the law did not achieve the desired result of reducing crime)

    As you can see in the last example, the opposite of effective is ineffective.

    If something is efficient, it means it is done in a good way, without wasting time, money, or energy:

  • E-mail is more efficient than regular mail. or ( e-mail saves time that is normally wasted with regular mail)
  • When I bought a more energy-efficient air conditioner, my electrical bill went down. (= the new air conditioner uses less energy)
  • That office is so inefficient – it took five weeks to process a simple document! (= that office wastes time and complicates matters)

    As you can see in the last example, the opposite of efficient is inefficient.