Weak

강약 = strength and weakness
약점 = weak point (점 = point)
약(하다) = to be weak
노약자 = old and weak people (giving up your seat for people on public transport (노약자석 = seat for old and weak people))

Contract

약속(하다) = promise, appointment (속 = bind) 
약혼(하다) = mariage engagement (약혼자/약혼녀 = fiancé (male/female))
계약 = contract
예약(하다) = make a reservation (예 = before)
요약(하다) = summarize, summary

Medicine

약 = medicine
약국 = pharmacy
약사 = pharmacist
약대/약학대학 = college of pharmacology
마약 = Drug
독약 = Poison, poisonous drug