The Additional Ingredients that you put into your potion can be broken down into three different categories:
1. Certain items will increase the yield that the recipe makes. In other words, you put in one set of ingredients and receive two sets worth of potions.
2. Other items will increase the duration of the potion's effectiveness. Instead of lasting 10 minutes, the potion may last much longer.
3. Other items still will increase the levels of stability in your potion. Too little of these stabilizers and your potion will become Volatile. No stabilizers and it will fail altogether.

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How dangerous is it to use a Volatile Potion? Many users have said that they've seen no real harmful effects from using them and that any they do see are easily outweighed by the relatively cheaper cost of making a Volatile Potion in comparison to a Stable Potion. However, there is risk involved. Volatile potions may fail or may even cause a negative reaction instead of the positive reaction the potion aught to give. It is easy to create a Stable Potion, though a bit pricier. Risking the use of a Volatile Potion is entirely up to the user.
For more Warhammer guides, please check out the links below.
Free Warhammer Online Guide: Cultivating
Talisman Making Basics: Your Free Warhammer Online Profession Guide