Can water-soluble vitamins taken in excess have a definite toxic effect?

Yes it can: Either water or fat-soluble vitamins, excess can cause problem. For instance, excessive use of vitamin c (water-soluble), can increase risk of kidney stones. Vit c also enhances iron absorption, thus problem for those having iron overload such as hemochromatosis. Moderation is key! why take these to excess? They may cause u health problem at the same time costing money. Consult doc. Good luck.