Excess B12 merely gets excreted by the kidney. There are vitamins which are potentially toxic. B complex vitamins are water soluble and generally are fairly non-toxic. However, Vitamin B6 has been reported to cause toxicity in very large doses. I would avoid industrial doses and keep to regular vitamin supplementation.
Vitamin A can be very toxic in high doses.
High doses can cause death. In general, Vitamins A, D, and K are fat soluble and may be toxic in high doses. Therefore, it is given only by prescription. The toxicity of Vitamin A depends on the age of the patient, the dose, and the length of time given. Toxicity has been reported at 10 mg a day ongoing, or 100 mg with one dose.