Conversation so far:
Did John Harvey Kellogg support drinking alcohol?
Alcoholic beverages were commonly used by the medical community during the time that John Kellogg began his medical practice. He was firm in his opposition to the practice.
Why was John Harvey Kellogg against alcohol?
John Kellogg believed that alcohol could not be a stimulant because it lessened vital activity and depressed vital forces. Kellogg also considered it to lead to mental and moral bankruptcy.
What other beverages was John Harvey Kellogg against?
John Kellogg also opposed tea and coffee due to the caffeine content of those beverages.
How did John Harvey Kellogg feel about coffee and tea?
John Kellogg's view was that caffeine was a poison.

Question: What did he feel was bad about using caffeine?
Self-contained rewrite: What did John Harvey Kellogg feel was bad about using caffeine?