efficacious
UK: ˌefɪˈkeɪʃəs | US: ˌefɪˈkeɪʃəs
adj. producing or capable of producing a desired effect; effective
The word "efficacious" stems from Latin efficax (effective), derived from efficere (to accomplish), combining ex- (out) + facere (to do/make). The suffix -acious (from Latin -ax) denotes a tendency or quality, giving the word its meaning of "capable of producing results." Over time, it evolved in English to specifically describe something that reliably achieves its intended purpose.
The new vaccine proved highly efficacious in clinical trials.
Her advice was simple but remarkably efficacious.
The treatment is efficacious for most patients within weeks.
They sought an efficacious solution to the recurring problem.
His leadership style was both calm and efficacious.