“What are the side effects of dental crowns?”

None

There are NO side effects to properly completed dental crowns. You do have to keep them clean and floss every day. Crowns are not like replacement parts in your car's engine, they are man made replacements for G-ds made crowns.
Crowns can be made out of porcelain, gold, zirconia, etc. Stay away from certain alloys as some people are sensitive or allergic.