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That medicine is efficacious in treating a cold
The word efficacious means about the same as "effective" -- it means capable of having the intended result.
Example:
"A shotgun is efficacious in controlling wild rabbits."
"For a simple cough, honey is often as efficacious as medicinal syrups."
(Capable of having a desired effect or result, effective.)
"The medicine is efficacious in stopping a cough."
"The farmers were looking for a chemical that would be efficacious against the corn fungus."
productive, efficient, effective
Quinine