Guthix rest is a tea, but its function is similar to a potion, as each cup provides several doses. It is made during One Small Favour from 2 guams, a harralander, and a marrentill. To make it, heat a bowl of water on a range. Use the bowl of hot water on an empty tea cup and add a harralander, two guams, and a marrentill, in that order. Making Guthix rest gives a total of 59.5 Herblore experience. If the incorrect amount of herbs is added, or the herbs are mixed in in the wrong order, ruined herb tea is obtained.
After One Small Favour is completed, the tea can still be made. It still gives Herblore experience and has several useful effects. It cures poison, heals 50 life points per dose, restores some run energy and boosts the maximum life points by 50 at full health. A full four dose Tea restores a total of 200 LP with the possibility of boosting the players lifepoints over their maximum level by 50.
When first released, there was a glitch that caused the tea to restore all of the player's health. The glitch was quickly fixed. However, many players were banned for abusing the glitch to gain an unfair advantage in PvP situations.
Attempting to make this tea by adding the herbs to the cup of hot water before speaking with Sanfew, even partially into the quest, will result in: "You have no idea what effect this would have."