Allows Isaac to carry two cards, runes, or soul stones at once. Cycle through cards by pressing the Control key (PC), R2 (PlayStation), RT (Xbox), or ZR (Switch, 3DS).
Replaces all pills with cards, runes, or soul stones. Pills already on the ground will be converted to a card, rune or soul stone upon picking up Starter Deck.
When picking up Starter Deck in a shop that has a pill for purchase, the pill can still be purchased and used. If the pill is dropped, it will be turned into a card.
Picking up 2 Starter Decks won't allow you to carry 3 cards at once.
Interactions
Acid Baby: Spawns cards or runes instead of pills. Using cards or runes will not trigger the room-wide poisoning effect.
Caffeine Pill: A pill drops normally, but if it is picked up and dropped, or left on the ground while visiting other rooms, it becomes a card.
Little Baggy: Overrides the effect of Little Baggy (switch from receiving only pills to only cards).
Having both items allow both cards and pills to appear again.
Picking up Little Baggy while already holding Starter Deck will turn any held cards or runes into pills for one time.
Starter Deck also appears in Cantripped! (challenge #43) with a unique function of increasing the number of item cards that Tainted Cain can carry at once. It replaces all Boss Room items in this challenge.
Acid Baby: Spawns cards or runes instead of pills. Using cards or runes will not trigger the room-wide poisoning effect.
Caffeine Pill: A pill drops normally, but if it is picked up and dropped, or left on the ground while visiting other rooms, it becomes a card.
Little Baggy: Overrides the effect of Little Baggy (switch from receiving only pills to only cards).
Having both items allow both cards and pills to appear again.
Mom's Bottle of Pills: Gives a card instead of a pill, essentially becoming equivalent to a 
Deck of Cards.
XIV - Temperance?: Forces Isaac to eat pills even with Starter Deck.
Tainted Cain can carry at once. It replaces all 
Boss Room items in this challenge.
Inventory Expansion 
The Binding of Isaac: Rebirth
