| n. | 1. | Lack of power to produce the desired or proper effect; inefficiency; ineffectualness; futility; uselessness; fruitlessness; |
| Noun | 1. | inefficacy - a lack of efficacy Synonyms: inefficaciousness Antonyms: efficaciousness, efficacy - capacity or power to produce a desired effect; "concern about the safety and efficacy of the vaccine" |