Definitions of effectual:

• adjective:   exerting or capable of exerting strong physiological or chemical effects

  Example: "An effectual vaccine"

• adjective:   sufficient to produce a result

  Example: "A man to whom painting was but another and less effectual way of writing dramas or novels or history"

• adjective:   producing or capable of producing an intended result or having a striking effect

  Example: "His complaint proved to be effectual in bringing action"