All trades are zero-sum. For any price change in a stock, the money made by one party is equal to the money that could have been made by the other. Given the

All trades are zero-sum. For any price change in a stock, the money made by one party is equal to the money that could have been made by the other. Given the choice to either buy now or buy later, if the price appreciates, buying later is a loss and if it falls, it is a gain. So in your example with the biotech stock, the person who loses the corresponding $100 is whoever sold it to you prior to the news event. If they had not made that trade, they would be $100 ahead, and if you had not made the trade, you would be $100 behind. Every transaction has a winner and a loser when considered over a period of time.