So this question came up while working on magic items. The "poisoned" condition is used as a general catch-all for being/feeling sick, not just that you got a dose of poison (most of which don't actually give the condition). Poisoned gives disadvantage on attacks and ability checks. While RAW this would likely apply to spells that make an attack roll, there is no mention of any other spellcasting, which seems kinda left out, as something mentally difficult especially when you don't feel good at all.
Does anyone else do something with this? Add an additional con save for casting/concentration?