Herbal Lore represents a character’s knowledge of herbs and natural substances. In order to mix an Alchemical elixir into a food or drink, or to apply a contact gel or weapon coating, you must have Herbal Lore.
Anyone can use an ingested substance if not mixed into food or drink. In other words, anyone can drink an Intoxicant elixir straight from the bottle, but you must have Herbal Lore in order to mix that elixir into a drink or food.
You cannot use the skill Herbal Lore on a character to determine if that character is under the effects of an alchemical substance.