Answer: The Gemara (Berachos 8a; Yoma 81b) writes that there is a mitzva to eat on erev Yom Kippur. According to Rashi, we do so to properly prepare for the fast on the following day (See Rashi, Rosh Hashana 9a).
The Sdei Chemed (Yom Kippur 1:18) writes that one mustn’t purposely continue eating until the fast begins as this will prevent one from feeling that they’re fasting on Yom Kippur. R’ Eliezer Waldenberg (Tzitz Eliezer 7:32:4), R’ Menashe Klein (Mishne Halachos 2:66:4; 7:82) and R’ Ovadia Yosef (Yabia Omer OC 9:54), however, challenge this, writing that we are not supposed to actively do anything to oppress ourselves on Yom Kippur. If we are supposed to eat and drink more before Yom Kippur, then we are allowed to plan our eating in this way. Thus, these poskim write that one may take slow release energy capsules before Yom Kippur (See Rivevos Ephraim 3:413:2). While R’ Ovadia Yosef writes that one should only take such pills if they really struggle to fast, other poskim allow everyone to take them.
Similarly, R’ Mordechai Yaakov Breish (Chelkas Yaakov OC:216) argues that taking such a pill is no different to eating well before Yom Kippur and deciding to eat again before the fast starts in order to curb their hunger pangs later. He adds, however, that it is important to ensure that the ingredients are all kosher. Only one who was ill would be allowed to take non-kosher medicine.
In conclusion, while some sefardim may avoid taking them unless they struggle to fast, the consensus is to allow taking such pills before the fast.