Medicine Buddha is a fully enlightened being. To understand who he is, what his nature is, what his function is, and so on, we first need to understand what an enlightened being is. Generally, 'being' means any being who experiences feelings – pleasant, unpleasant, or neutral. Thus we are beings, and animals are beings; but houses and plants are not beings because they do not experience any feelings. There are two types of being: sentient beings and enlightened beings. A sentient being, or living being, is a being whose mind is afflicted by the darkness of ignorance. An enlightened being is a being who is completely free from the darkness of ignorance. 

Just as sentient beings have many different aspects, so do enlightened beings. Enlightened beings emanate countless different forms for the benefit of living beings. Sometimes they appear as Deities, sometimes as humans, sometimes as non-humans. Sometimes they appear as Buddhist Teachers, sometimes non-Buddhist Teachers, sometimes as crazy people or evil people, and sometimes even as inanimate objects. Emanations of enlightened beings pervade the whole world, but because our mind is covered by ignorance we do not recognize them. We cannot say who or what is an emanation of a Buddha. 

Medicine Buddha is an enlightened being who has unbiased compassion for all living beings. He protects living beings from physical and mental sickness and other dangers and obstacles, and helps them to eradicate the three poisons – attachment, hatred, and ignorance – which are the source of all sickness and danger. He is a Buddha Doctor. 

At one time Buddha Shakyamuni was staying at a place called Vaishali with thirty-six thousand Bodhisattva disciples. At that time, Manjushri was appearing as a Bodhisattva disciple. Through his compassion, Manjushri realized that in the future the Buddhadharma would degenerate, and the beings of this world would find it very difficult to practise pure Dharma and gain pure realizations. He understood that it would be very difficult for those beings to control their minds, and so they would naturally engage in negative actions such as killing, stealing, and holding wrong views. As a result they would experience horrific illnesses and unbearable mental pain. The world would be full of problems, dangers, and adversity. Finding the thought of all this suffering impossible to bear, Manjushri asked Buddha:

In the future when your Dharma and general spiritual practice are in decline, when the human beings in this world are spiritually impoverished, when their attachment, anger, and ignorance are so strong and difficult to control that they experience continual physical suffering, mental pain, fears, and dangers, and especially many incurable diseases, who will release them from this suffering and protect them from danger? Who will help them to overcome the three mental poisons?

In response to Bodhisattva Manjushri's question, Buddha expounded the Sutra of Eight Thousand Verses Principally Revealing the Instructions on Medicine Buddha. Many beings heard this teaching. In addition to the thirty-six thousand human Bodhisattva disciples, millions of other Bodhisattva disciples came from many Pure Lands, together with beings from other realms such as nagas and givers-of-harm, or yakshas. To this vast assembly of disciples Buddha explained all about Medicine Buddha - his special qualities, his Pure Land, and how in the future by relying upon this Buddha and just hearing his name, living beings could be cured of heavy mental and physical sickness, especially the sickness of delusions. He also explained how to make a connection with this Buddha, the benefits of relying upon him, and how to practise the Medicine Buddha instructions. 

While Buddha was giving this teaching, Manjushri realized with his clairvoyance of knowing others' minds that some of the humans and gods in the audience were developing doubts, finding it difficult to believe the Buddha's explanation about the existence of Medicine Buddha. Therefore, again he rose from his seat, respectfully circumambulated Buddha three times, made three prostrations, and then with his left knee on the ground according to tradition, requested Buddha:

To remove doubts from the minds of disciples, please show clearly how this Buddha exists, where he exists, and what his good qualities are.

Buddha immediately entered into an absorption of concentration, and from his heart emanated light rays inviting the seven Medicine Buddhas to Vaishali so that everyone could see them. Medicine Buddha came with his two main disciples, Radiance of the Sun and Radiance of the Moon, as well as a vast retinue of thousands of other disciples. The other six Medicine Buddhas also came with their retinues. Everyone could see the seven Medicine Buddhas with their retinues directly, and their doubts were immediately dispelled. Buddha introduced each of the Buddhas, saying for example "This Buddha is Medicine Buddha. He comes from the eastern Pure Land called Lapis Jewel Land. This Buddha Land is the nature of wisdom with the aspect of lapis lazuli. The entire ground of that Pure Land is illuminated by this Buddha's light," and so on. 

Buddha then gave instructions on how to recite the mantra for oneself and for others, for sick and dying people, and so forth, and how to perform many different healing rituals. Everyone rejoiced and developed deep, unchangeable faith. It is said that through hearing these instructions seven million non-human givers of harm gained a direct realization of ultimate truth and promised to help future followers who sincerely relied upon the practice of Medicine Buddha. Twelve chief givers-of-harm who were present later attained enlightenment, and are included within the fifty-one Deities of Medicine Buddha’s mandala. 

The practice of Medicine Buddha is a very powerful method for healing ourself and others, and for overcoming the inner sickness of attachment, hatred, and ignorance. If we rely upon Medicine Buddha with pure faith we shall definitely receive the blessings of these attainments.