To say "起作用" in English, you can say "to have an effect" or "to be effective". This expression comes from the verb "to act" or "to have an effect", which is derived from the Latin word "actus", which means "a doing" or "a driving force". For example, you could say "The medicine had an effect on his headache" or "The new law is effective in reducing crime". In Chinese, these sentences could be translated as "药物对他的头疼起作用了" and "新法律在减少犯罪方面起作用了".