"White Honey strongly stimulates the production of purifying enzymes in witchers' mutated bodies. Thus, it frees their bodies of the effects of toxicity, though it simultaneously nullifies the beneficial effects of any potions previously consumed. White Honey does not alleviate the effects of common venoms and poisons."
White Honey reduces Toxicity to zero and cancels the effects of other potions.
White honey is useful in situations where you don't have readily access to a fire, but urgently need to drink more potions - typically in-between intense combat. However, since it also removes existing potions effects, it is best used together with short-duration or instant effect potions, such as Blizzard or White Raffard's Decoction.
If White Honey contains a Secondary Substance the effect WILL be applied and not be removed unless you drink another White Honey after. So for example a Rubedo dominant White Honey can be used as a weak albeit completely non-toxic healing potion. Also since White Honey is often used when intending to drink other potions it is a good idea to create Albedo dominant White Honey, as while being non-toxic it will not benefit from the effect itself it will however lower the toxicity of the potions you intend to drink right after.
Geralt has the ability to brew this potion from the moment he arrives in White Orchard. Only one of the basic potions can be crafted at any one time and placed in the consumables slot, however this increases with each of the higher level variants.