Extracting

When an alchemist doesn't use his spiritual energy to purify an ingredient and instead directly uses fire to burn it, they successfully kill impurities present in its outer surface but not those present deeper inside it.

The leftover impurities seep into the deepest part of an ingredient, where lies its medicinal essence.

What happens then?

The impurities become one with the medicinal essence and corrupt its purity.

When a person uses such medicinal essence to refine a pill, what he creates carries toxins within it.

If a person eats such pills in large quantities, the toxins build up inside them, and when it reaches an amount their body can not endure, they fall ill, sick, and even worse, find their cultivation base dangling on the verge of destruction.

People who use an enormous amount of pills to advance their level usually find their elixir field breaking down or their spiritual core shattering when the toxins they have suppressed in their body erupts.