I would expect it to have some effect on the Yeerk, being neurologically linked with the host. We can be sure there would be some long term mental effects, since it's shown that host mentality does effect Yeerks. Hork-Bajir controllers are shown as kind of dumb, Taxxon controllers can be as violent as unhosted Taxxons, Human controllers can be more emotional, and we know Visser Three is bat**** insane. If a Yeerk spent its life with a drug addled host, it would be effected one way or another.