In my fantasy Mana is a resource found not only in people (really, really, really trace amounts) but also physically in the ground. In its natural state Mana is a bright blue stone that at night glows intensely however the only known source of Mana is deep within the Endless Sea.
Mana is, of course, a magical substance. In close proximity to Mana one has powerful dreams and visions, but if someone consumes Mana those dreams can become true. However, I've realized that allowing Mana to basically grant miracles can get out of hand and I don't want Mana to become Phlebtonium.
What could be some effects of Mana usage that would prevent long term abuse?
What I'm looking for in a answer: my two main requirements are that it's related to a person's mind/psyche (physical effects could be included but are not necessary) and second the dreams are tied into the effect, however I'd prefer the dreams aren't what directly kills them ex: they dream about falling and actually fall isn't something I'm looking for.
being near Mana and having powerful dreams would have minimal side effects compared to actually consuming Mana. So only after years of direct contact with Mana (not consumption) would the user feel adverse effects.
inhaling, grinding into dust, mixing into a liquid, are all valid ways to consume Mana. It's why mining Mana is a dangerous pursuit.