Before we try to answer a very generic question whether vitamins are toxic or not, let us take a look at some definitions first.
Vitamins are normally defined as essential nutrients which guarantee the healthy function of the human body. In other words, the human body cannot function properly without sufficient amounts of vitamins, as well as other nutrients - ex., minerals & trace elements.
Vitamins have to come from foods because the human body, unlike many animals, cannot synthesise them.
There are different forms of vitamins which are often confused with each other. To put it simply, we have:
1. Natural vitamins. These are the vitamins found naturally in foods we consume. They are complex compounds that comprise of many different chemicals. These chemicals come together to deliver biological functions of each specific vitamin inside the human body.
For example, according to Wikipedia, vitamin A is a group of organic compounds which includes retinol, retinal, retinoic acid and several carotenoids. (Ref. 1)
Vitamin C, according to Dr. Ben Kim, is made up of rutin, bioflavonoids, factors K, J and P, tyrosinase, ascorbinogen, ascorbic acid. (Ref. 2)
And natural Vitamin E contains 8 chemically distinct molecules: α-, β-, γ- and δ-tocopherol; and α-, β-, γ- and δ-tocotrienol.
So, natural vitamins that are found in real foods come in the form of complicated inter-connected structures, and not just as isolated chemicals existing independently of each other.
2. Synthetic vitamins. These vitamins are primarily synthesised in laboratories. Most of the time, such vitamins are isolated chemicals which usually don't form any inter-connected biological compounds but simply exist independently of each other.
Most people associate vitamins with such synthetic vitamins - exactly as sold in supermarkets.
3. Various combinations of natural vitamins and synthetic vitamins. As the industry of nutritional supplements has grown exponentially over the last couple of decades, you can now find literally hundreds of different combinations of natural and synthetic vitamins, with different levels of nutrient potencies.
Our body absorbs vitamins properly when they are presented in the correct form - as organic compounds, with all other accompanying chemicals which make it easy for the body to process them.
If we consume healthy foods in appropriate amounts, there will be very little waste from this process of vitamin absorption. In this case, the body simply won't produce much toxic waste.
From this point of view, natural vitamins - as consumed from living fruits and vegetables - simply cannot be toxic.
In addition to that, natural vitamins are always present in small amounts - delivering just the quantities that are required by our systems, no more and no less. Our intuition usually tells us how much of each food to take.
Synthetic vitamins, on the other hand, require other nutrients to help them get properly processed by the body. So, synthetic vitamins draw on the body's existing nutrient reserves forcing it to divert its valuable resources to the processing of synthetic, isolated, dead vitamins. In the long-term, this may lead to synthetic-vitamin overload and toxicity.
Some researchers even mention possible liver toxicity (hepatoxicity) as a result of excessive intake of certain synthetic vitamins. (Ref. 3)
Hyper-vitaminosis is a phenomenon of modern age. It refers to an excessive storage of synthetic vitamins inside the body - leading to toxic effects.
Frequently, a significant percentage of synthetic vitamins is simply excreted from the body via urine or stools, without being properly assimilated. It is a well-known fact that laboratory-synthesised vitamins have low levels of bioavailability. Probably because we, the humans, cannot compete with Nature.
I would also like to mention here that, ironically enough, more toxicity can be caused by all sorts of additives which are used alongside synthetic vitamins. For example, magnesium stearate is a flow agent and allows equipment for making supplements to run smoother but has no nutritional value at all. It is believed to disrupt the function of the immune system as well. (Ref. 4)
If you still have to take isolated vitamins for one reason or another, bear in mind that some synthetic vitamins are potentially more toxic than others.
1. Water-soluble vitamins, ex. vitamin C, are unlikely to be very toxic to the body, when taken on their own even in excessive amounts - because they stay in the body for a short period of time.
2. Fat-soluble vitamins, ex. vitamins A, D & E - can be toxic if taken in large amount because they are stored in the body for longer periods of time than water-soluble vitamins.
These days, there is an enormous variety of all sorts of different vitamin supplements. Some will always be better than others.
I have come across many great companies that do their utmost to offer additive-free vitamin supplements.
Nonetheless, I have a strong opinion that in most cases raw & whole foods are the best sources of natural vitamins we can think of.
1. Vitamin A (June 25, 2014). In Wikipedia, The Free Encyclopedia. Retrieved June 27, 2014 from: https://en.wikipedia.org/w/index.php?title=Vitamin_A&oldid=614351682
2. Dr. Ben Kim (2014). Synthetic vs. Natural Vitamins. Retrieved June 27, 2014 from: https://drbenkim.com/articles-vitamins.html
3. Shi-Sheng Zhou and Yiming Zhou (February 15, 2014). Excess vitamin intake: An unrecognized risk factor for obesity. Published by World Journal of Diabetes. Retrieved June 17, 2014 from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3932423/
4. Dr. Joseph Mercola (June 23, 2012). Does Your Supplement Contain this Potentially Hazardous Ingredient? Retrieved June 27, 2014 from: https://articles.mercola.com/sites/articles/archive/2012/06/23/whole-food-supplement-dangers.aspx
5. Chandan K. Sen, Savita Khanna, Cameron Rink, and Sashwati Roy (2007). Tocotrienols: The Emerging Face of Natural Vitamin E. Published in Vitamins & Hormones. Retrieved December 6, 2014 from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3681510/