What is another word for "efficacy"?

Another word for efficacy is effectiveness, which is the most common and direct synonym, meaning the power to produce a desired result. Other synonyms depend on context but include potency which emphasizes strength, particularly for medicines or chemicals, power, strength, influence, capability, capacity, ability to produce effect, efficiency in some contexts though efficacy and efficiency have distinct technical meanings, usefulness, productiveness, success, virtue in older usage meaning inherent power, and performance. In medicine you might say therapeutic efficacy could be replaced with therapeutic effectiveness or clinical potency. In general business or technical writing you could say the efficacy of a method could be described as its effectiveness, its ability to deliver results, its success rate, or its impact. For example instead of saying we measured the efficacy of insulation you could say we measured the effectiveness, performance, or ability to reduce heat loss. In formal writing you might use efficaciousness, though that is less common. Choosing the right synonym depends on whether you want to emphasize power, success, or practical result, but effectiveness is universally understood as equivalent to efficacy.