External substances depend entirely upon how one imbibes them or uses them. An alchemist experimenting with anything and everything he can in pursuit of some new arcane formulae is likely to do as much if not more harm than a spiritual advisor, shaman, sage or seer; someone who is tied to the understanding and well practiced in the gathering, refining and dispensing of altering goods. No less, it keeps the practice secret, limited and word-of-mouth, passed on from mentor to pupil within a tribe or people. When you go on a "spiritual journey" or "dreamwalk" through medicinal herbs and a bit of magical tact, you get a radically different experience than simply quaffing down a potion the local apothecary has thrown together based on some book. One is a positive, enlightening experience and the other is... varying, depending on any number of factors. However...

Magic ceases being magical when you begin to refine it down to a science. Even mighty wizards, who spend their entire lives pursuing the art, are not always so certain why some things do as they do. Forcing it into whirring cogs, grinding gears, and machines turns it into a science. It removes the mystery of it all, what makes it arcane or complex for a number of reasons, not the least of which is availability. When you turn magic into any other tool, any fool can then wield it. You keep magic limited to liveliness and as much art as it is science, you make people with it distinct. It needs to be a power that as one grows in scale of a spell, the more wild and awesome it becomes; world changing magic does just that, but often in unpredictable ways.

A mage might be proficient enough to cast lowly bolts of flame without danger, but a great wizard who has been preparing a spell for many, many long years? He might indeed cross a planar boundary and travel ahead or back in time, but the sheer power of the magic unleashed could let other things in too, or cause great harm to the surrounding environ. You can never be too sure. He can check and recheck his books, calculations and studies, but there [i]is[/i] always that chance. Yet, he is a wizard so that is but only a chance, [i]right[/i]? 

On another note, I am very fond of the concept that people who attempt to "study" magic begin to edge closer to madness the deeper they dive, whereas those born into the talent adapt with it. A wizard contrasting a sorcerer; the absent minded, aged man with beard and innumerable studies who bumbles and babbles to the keen, almost seductive beauty and power that someone was just born with and commands on whim. A sort of envy arises from that and a desire to pursue it.

Returning back to the subject of cats which drew me here, felines of all forms have a great number of magical mythos and properties associated with them across a number of cultures. If there's any animal one might think to be a danger to magic and mages, even the common house cat is an option; they're said to see spirits, receive visions, steal away life itself or breath it into others, balance the power of plagues and illness, chance or change fortune, among other things. Not to mention they are a very classical familiar to the scholarly, cloistered and magically inclined. Your larger cats? Some are informally called "ghosts of the mountains", others are divine spirits, representing everything from unchecked, pure and impartial wrath like the tiger, up to keepers of secrets, tricks or mystery as with a lynx. 

If there is something a mage might not want to dabble in, that would be a terrifying mythic or enchanted beast, that would be any cat.