Serum Powder — ruling 1

You can use Serum Powder's second ability only while it's in your hand. If this card is in your hand, you can choose either to mulligan or to use Serum Powder's ability. Using the ability doesn't prevent you from taking further mulligans, and taking a mulligan doesn't prevent you from using a Serum Powder's ability if you happen to draw one in your new hand.

Serum Powder — ruling 2

The cards in your hand are exiled for the rest of the game; they aren't shuffled back into your library if you take another mulligan. Cards exiled are always face up unless the effect that exiled them says they aren't. Be sure to return those cards to your deck for your next game.

Serum Powder : règles, fonctionnement et interactions (MTG)

Serum Powder is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

: Add . Any time you could mulligan and this card is in your hand, you may exile all the cards from your hand, then draw that many cards. (You can do this in addition to taking mulligans.)

How it works : Serum Powder

Original English text — Wizards of the Coast / Scryfall

#1November 17, 2017· Wizards of the Coast
You can use Serum Powder's second ability only while it's in your hand. If this card is in your hand, you can choose either to mulligan or to use Serum Powder's ability. Using the ability doesn't prevent you from taking further mulligans, and taking a mulligan doesn't prevent you from using a Serum Powder's ability if you happen to draw one in your new hand.
No other card is named in this ruling.
#2November 17, 2017· Wizards of the Coast
The cards in your hand are exiled for the rest of the game; they aren't shuffled back into your library if you take another mulligan. Cards exiled are always face up unless the effect that exiled them says they aren't. Be sure to return those cards to your deck for your next game.
No other card is named in this ruling.