Handheld games:
Generation III:
A Sitrus Berry, if held by a Pokémon, can restore 30 HP when its HP drops to below ⅓, or can be used as an item to restore 30 HP at any time.
Generation IV:
When held by a Pokémon, a Sitrus Berry will restore ¼ of its HP when its HP drops to below ½. It can also be used as an item to restore ¼ of a Pokémon's HP at any time.
Generation V
When held by a Pokémon, a Sitrus Berry will restore ¼ of its HP when its HP is below ½. It can also be used as an item to restore ¼ of a Pokémon's HP at any time. . .
PRO:
A Sitrus Berry, if held by a Pokémon, can restore 25% of max/HP when its HP drops to below ⅓.
It can also be used as an item to restore 30 HP of a Pokémon at any time.