For medicine to be effective; the medicine works to relieve or cure symptoms.
風邪薬を飲んだらすぐに薬が効いた。
After taking cold medicine, it worked immediately.
薬が効かないときは医者に相談したほうがいい。
If the medicine doesn't work, you should consult a doctor.
この薬は頭痛に特によく効くと言われている。
This medicine is said to be especially effective for headaches.