The lifebone pill was something Old Yao refined using the lifebone flower and possessed a much gentler medicinal power. Even so, the medicinal power was still boundlessly powerful for Guo Po.

The lifebone pill was a pill greatly effective at treating muscles and bones. It would be useful to even Saints. Thus, it was understandable that an Astral Realm kid would have a hard time enduring the medicinal power. 

As the lifebone pill erupted with its medicinal power, Guo Po's bones were reforged. The pain of having new bones growing in him caused him so much pain he nearly fainted.

"Guard your mind, circulate your energy through your meridians, gather them in your stars, and transform the energy into new bones,"...