look on this quest guide: http://runescape.wikia.com/wiki/Witches_Potion
It is a quest, that introduces a new potion into the game, and is a prerequisite for other quests.
Makes a person appear dead. This potion was only useful in the Romeo and Juliet quest, which was removed from RuneScape and replaced by another quest. If you have your character drink it, he/she appears to faint and reawakes in a moment or two.
The Blood Pact, Witch's Potion, Farmer ted's quest, and cook's assistant.
The apothecary in runescape is located in southwestern varrock. it has a purple potion icon over it.
To solve the Romeo and Juliet quest on Runescape, you will need to talk to Juliet, speak to Romeo, get them a cadava potion, and resolve their love story. The quest involves a mix of talking to NPCs, gathering items, and completing tasks to progress through the storyline. Follow the quest guide in-game for step-by-step instructions.
It's currently not possible to undo a quest in RuneScape.
The legends quest on runescape is not gone, I suggest that you check again.
A quest on Runescape is an adventure that puts you in a conflict, you solve it and you get rewards like money, skill experience points, quest points, and items.
There are several green coloured potions in RuneScape, but there is a non-drinkable potion just called 'Potion' which is a free to play item which I got from a drop from an Imp. They don't really have a main purpose, the only time I ever needed to use it is for was a member quest called Forgettable Tale of a Drunken Dwarf for when the Rowdy Dwarf seems to ask for a random item. My best advice is to save bank space and drop it. =)
If it's part of a quest, you need to look underneath one of the plant pots outside for a key to the house, make sure that the witch doesn't spot you once your in the garden.
If you quit runescape...
There are many websites where one can get advice on completing a Runescape quest. The Runescape website itself offers advice as does the website Tip.It.