Casting in DD5 uses your class's spell casting ability for the range attack.
So as a Wizard it's an Range check with your Intel bonus. When a spell says constitution like poison, it refers to the enemy saving throw not yours .
That helps, I've dumped Str and Con on my Wizard then. I put 14 in Con, Wis and Cha to help with HP saves and social things and also took the friends cantrip which might be useful later.
My poison spells would be having a lower chance against enemies with high Con then.
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