When the time for Zakhor – fulfilled through kiddush – arrives, it is a mitzva to fulfill it with alacrity. So that this mitzva is not neglected, the Sages prohibited eating or drinking, even just water, before kiddush. However, one may wash out one’s mouth or swallow medication with water (SA 271:4; MB 13; SSK 52:3).
On Friday night, this prohibition applies the moment Shabbat begins. Therefore, a woman who accepted Shabbat when lighting the candles may not drink until she fulfills the mitzva of kiddush. A daughter who does not light candles must accept tosefet Shabbat by saying “Bo’i kalla Shabbat Ha-malka,” and from that point on she may not eat or drink until kiddush. Likewise, a man who accepted tosefet Shabbat may not eat or drink until he fulfills the mitzva of kiddush (MB 271:11; See Peninei Halakha: Shabbat 5:9).