noun
statement of efficacy; list of benefits
Refers to a written description of the beneficial effects of a medicine, supplement, or health product. Often found on packaging or in advertisements.
この薬の効能書きには、肩こりに効くと書いてある。
The statement of efficacy for this medicine says it works for stiff shoulders.
効能 means 'efficacy' or 'beneficial effect' itself, while 効能書き is the written document or statement describing those effects.
Compound of 効能 (こうのう, 'efficacy') and 書き (がき, 'writing' or 'document').