Health Tonics are special potions Plague Knight can drink in Plague of Shadows. Each Health Tonic drank increases Plague Knight's maximum hit points by one green Health bubble up to a limit of twenty and also refills his Health by two hit points. If some Health is missing and the full capacity of the Tonics have not been reached, each Tonic will heal the wounds and add a green bubble, albeit an empty one, up until all wounds are healed, the maximum number of green bubbles has been reached, and all of them have been filled. Even if Plague Knight's Health has already reached its maximum capacity, he can still uses the Tonics as healing items. Upon death, all increased Health Capacity granted by Tonics is lost, which makes Pits and spikes very undesireable, resource-costly hazards for Plague Knight.
Plague Knight can initially hold only five spare Tonics at any given time and applies them from the Alchemy Screen. However, by buying the Big Tonic Bag, he can double the quantity, holding up to ten spare tonics.
Health Tonics can be found in Dig Piles, Fishing Spots, inside food platters or in some large chests (where usually Relics are found in). Like treasures, Jars and Food, the Health Tonics are magnetized by the Treasure Trappings. Any Health Tonic that was not originally out in the open when first found (digged out, fished...), will eventually blink out of existence if not collected quickly. Health Tonics found inside dig piles and platters will respawn where they were first found upon death, but not the ones fished from sparkling pits, which also will permanently disappear if they fall again in the pit they came up from. The large chests containing a stash of Health Tonics will not refill twice during the same save file.
In addition Plague Knight can refill his entire Tonic reserve from the Magicist, at the cost of 100 gold per Tonic or, if Baz was recruited, he will allow Plague Knight to refill for free with Tonics he stole from the Magicist, but only once upon completion of a stage.
- There is usually an hidden alcove with a food platter and an Health Tonic before each boss. With this in mind, should Plague Knight perish against the boss and lose all of extended Health, it is possible to restock some Tonics before starting the boss again.
- Strategically, its possible to purposely die on the boss to respawn, re-obtain Tonics, and repeat until they're maxed out, possibly starting a boss fight again with maximum Health and healing items. However, this will only work if the Tonics are not drank before the process is completed. In New Game Plus neither the food nor the Tonic are present in the alcove, making it impossible to use this trick.