Marijuana
ConfirmedDocumented in a guide or agreed on by players who tested it.
Smoked from the player's inventory. The specific gameplay effect is not documented.
Like beer, this is confirmed to exist and to be usable from the inventory by the developers' own content descriptor on Steam. The gameplay effect is not published anywhere.
It matters far beyond its effect. Australia's classification rules treat drug use tied to incentives or rewards strictly, and Halloween: The Game is not purchasable on Steam in Australia — a difference you can verify yourself against the store API, and one that does not apply to New Zealand despite what a lot of coverage says.
Where this comes from
Steam content descriptor for appid 3219630: "Players can consume beer and smoke marijuana from their inventory." First-party, submitted by the developer.
How to read this
Every item page on this site carries one of three states, and they mean different things. Confirmed means a specific, actionable effect is documented or agreed on by players who tested it. Theorised means a hypothesis exists and the people offering it say it is unproven. No published effect means the item exists and is findable and nobody has published what it does — which is a real answer to a real question, and the one most likely to be dressed up as something else elsewhere.
Keep going
Ancient Amulet
Negates Michael's first grab and blinds him for roughly three seconds — no button press needed.
Fragile Hourglass
Players think it does something to Michael's cooldowns. Nobody has confirmed it.
Brass Lantern
A rare item with no published effect. Named by players as one the game never explains.
Sources: Steam store listing (app 3219630)