Buy OSRS Extended super antifire(3)
The Extended super antifire(3) is a potion in Old School RuneScape with three doses. Each dose gives full immunity to dragonfire for exactly six minutes. That means one 3-dose potion covers 18 minutes of protection. You must start the Dragon Slayer I quest before you can drink it.
This potion is great for fighting dragons. With full dragonfire immunity, you do not need to rely only on a shield to block the flames. It has some limits though. It does not block Galvek's attacks. It does not stop the King Black Dragon's poisonous, shocking, or freezing breath. It only partly blocks Vorkath's attacks.
The game warns you before the effect ends. A chatbox message says your super antifire potion is about to expire. About 15 seconds later, a second message tells you it has expired. This makes it easy to know when to drink another dose.
Making this potion yourself is not easy. It takes 98 Herblore and lava scale shards mixed into a super antifire potion. You also need to learn the recipe from Primula in the Myths' Guild, which needs Dragon Slayer II. Buying it saves all that work.