First solution so please bare with me :)
Easy way is to start up a single player mode, when in game press the start button and at the top of screen you will see a bar and "admin command" click the bar and type in the code for the item you want to spawn in.
LESSER ANTIDOTE -
cheat giveitem "Blueprint'/Game/PrimalEarth/CoreBlueprints/Items/Consumables/BaseBPs/PrimalItemConsumable_CureLow.PrimalItemConsumable_CureLow'" 1 0 0
DISEASED LEECH -
cheat summon Leech_Character_Diseased_C
When the leech takes hold of you, in the bottom
Left corner a hazardous sign will appear which is a sign you are infected, now just jump on top of a fire to burn the leech off and use the antidote.