From Bulbapedia, the community-driven Pokémon encyclopedia.
Herbal medicine is a type of item from the Pokémon games. They can be used during or out of battle to restore a Pokémon's HP, remove status conditions, or revive a Pokémon, all with a side effect of lowering a Pokémon's happiness due to their bitter taste. They can be found on the ground, whether readily visible or hidden from sight and requiring an Itemfinder to locate; however, herbal medicine is usually bought from special herb shops.
Types of herbal medicine
Costs 500 at Herb Shops. Restores 50 HP.
Costs 800 at Herb Shops. Restores 200 HP.
Costs 450 at Herb Shops. Removes all status conditions.
Costs 2800 in Poké Marts. Revives a fainted Pokémon to half of its HP. Starting in Generation IV, instead revives a fainted Pokémon to full HP.
The effect of lowering a Pokémon's happiness due to its bitter taste applies even to those with a Calm, Gentle, Sassy, or Careful nature, which are supposed to prefer bitter taste. This may be due to the fact that herbal medicine was introduced in Generation II, while natures weren't introduced until Generation III.