User blog comment:Iam69god/Why is "Lost magic" lost?/@comment-29693708-20160908224005/@comment-1776824-20160909002759

Not all lost magics have side-effects. For instance, God Slaying Magic is without any, as is Ushi no Koku Mairi. Arc of Time is without it as well. For the most part, lost magics became lost solely because, as time went on, less and less people able to perform the magic, and teach it, were around. For instance, Dragon Slayer Magic became lost due to dragons dying, so its effects were replicated with lacrima. Ushi no Koku Mairi is just outright forbidden, and others with side effects just don't have pros that outweigh cons.