effective, efficient

Effective refers to an action, a person or a device that produces a desired result.

• Despite her lack of experience, the Minister proved highly effective in her new job.
• The medicine is effective even in the advanced stages of the disease.

Effective also means “actual” or “having the force of law.”

• The commander of the armed forces was the effective (or true/actual) leader of the nation.
• When the legislation becomes effective, manufacturers will no longer be able to market this product.

Efficient refers to the skilful use of time and energy to produce desired results with minimal effort.

• To help consumers cope with increased energy costs, designers have made car engines more efficient.