効 (Kanji for effective)

Onyomi

KOU	COLONS are very effective for discharging waste.

If you combine the strong points of various techniques, you will get the most effective style!

( xxxが ) き＊く

xxx is effective, most often said about medicine or drugs.
★★★★☆

効果的(こうかてき) な	effective ★★★☆☆ 効果 (an effect) + 的 (motivation / '-al') = 効果的 (effective)
効果(こうか)	an effect ★★★☆☆ 効 (effective) + 果 (fruit　/ result) = 効果 (an effect)
効率　が　いい(こうりつ　が　いい)	efficient ★★☆☆☆ 効 (effective) + 率 (ratio) = 効率　が　いい (efficient)
逆効果(ぎゃくこうか)	the opposite effect ☆☆☆☆☆ 逆 (opposite) + 効果 (an effect) = 逆効果 (the opposite effect ) the opposite effect from what you'd intended.