It is forbidden to eat or drink anything before Kiddush, both on Friday night and on Shabbat day. As soon as the obligation of Kiddush takes effect, one may not eat or drink until he recites or hears Kiddush. This prohibition applies even to drinking water, as noted by the Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909).
When does the Kiddush obligation take effect?
On Friday night, the obligation begins at sunset. Therefore, if a person comes to the synagogue on Friday evening, and during the Kabbalat Shabbat he wants to take a bite or a drink, he may not do so. Since the obligation of Kiddush had set in at sundown, he may not eat or drink anything until he recites Kiddush. On Shabbat morning, the Kiddush obligation begins after one has recited Shaharit. Before Shaharit, the Kiddush obligation has not yet taken effect, and so the prohibition against eating or drinking does not yet apply. Of course, Halacha in any event forbids eating before praying Shaharit in the morning. However, it is permissible before praying in the morning to drink water, and many authorities allow drinking tea and coffee, as well. It is therefore permissible to drink these drinks before Shaharit on Shabbat morning, just as on weekdays, since the Kiddush obligation does not take effect until after Shaharit. After Shaharit, however, such as during the Torah reading or during Musaf, one may not eat or drink anything, even the drinks mentioned above.
When does the prohibition of eating and drinking before Kiddush take effect for women?
If a woman generally recites Shaharit each morning, then her status in this regard is no different than a man’s. She, too, may drink water and the like in the morning before Shaharit, and once she recites Shaharit she may not eat or drink anything until she recites or hears Kiddush. However, some women do not recite Shaharit each morning, and recite Minha or Arbit instead. For them, the Kiddush obligation takes effect as soon as they arise in the morning. They may therefore not eat or drink anything – including water, coffee and tea – from the time they wake up until they recite Kiddush.
It should be noted that Hacham Ovadia Yosef allows a person to drink water before Kiddush if he feels very thirsty. Although one should generally refrain from drinking even water before Kiddush, in situations where a person experiences considerable discomfort due to the thirst he may be lenient and drink water before Kiddush.
Summary: On Friday night, one may not eat or drink anything from sunset until he recites or hears Kiddush. On Shabbat morning, one may drink water, tea and coffee before Shaharit (just as on ordinary weekdays), but after reciting Shaharit it is forbidden to eat or drink anything until Kiddush. Women who do not recite Shaharit each morning may not eat or drink anything from the moment they wake up on Shabbat morning until Kiddush.
