According to Yakushi hongan kudokukyo, Yakushi Nyorai was a founder of a religious sect in Toho joruri sekai (Eastern Pure Land), and swore 12 oaths when it was Bodhisattva, lengthened people's lives by curing their diseases, removed disasters, provided clothes and food, practiced Buddhist ascetic training and finally became Buddha by attaining Mujo bodai (perfect Buddhist enlightenment).
薬師本願功徳経では、薬師如来は東方浄瑠璃世界の教主で、菩薩の時に12の大願を発し、この世門における衆生の疾病を治癒して寿命を延べ、災禍を消去し、衣食などを満足せしめ、かつ仏行を行じては無上菩提の妙果を証らしめんと誓い仏と成ったと説かれる。