Essence Supplement is a nutritional supplement for Monster Girls. It's processed from essence collected by Golems and noted to be capable of replenish mana with higher efficiency than ordinary meals, yet has a reputation of being extremely unappetizing. The supplement's first mentioned in Monster Girl Encyclopedia World Guide I: Fallen Maidens and later described in detail in Monster Girl Encyclopedia World Guide III: Sabbath Grimoire.
Guide Book Entries
World Guide I: Fallen Maidens
[...] Additionally, there are also nutritional supplements made using energy collected by "golems". They're drugs for monsters that can't get a human man, and a tiny amount replenishes a huge amount of energy compared to normal food, but since this energy that's not obtained through sex with men is bland and doesn't taste very good, it's basically not preferred. It tastes especially disgusting to monsters who have tasted their “husband's mana” at least once as described before, and it's not something they can bring themselves to drink. [...]
World Guide III: Sabbath Grimoire
A powdery medicine of white powder, or a hardened solid medicine, that can replenish mana when monsters swallow it. By processing the essence of men collected by "Golems", keeping in mind an unspecified number of monsters drink it, the individual male's information included in the mana is removed, and pure "Human Mana" is made. Although the efficiency and quality is far inferior to that obtained from men through intercourse, it can replenish mana immediately with higher efficiency than ordinary meals, and is a useful item to magicians run dry on mana, or as a nutritional supplement when exhausted.
However, this medicine has a reputation of being extremely unappetizing amongst monsters. To monsters, a man's essence is a first-rate treat, but what is more delicious than anything else, in addition to the taste of the essence itself, the taste peculiar to the individual male included in it, and the flavor added through the act of intercourse is lost. And above all else, due to the strong feelings monsters have towards men, their husband's essence will be even more delicious. This "Essence Supplement" in which none of that is present, is thin-tasting itself, and because of the extremely unpleasant sensation of it being tasteless, even though it should be their favorite food of essence, it is, generally speaking, a substitute which can only be said to be disgusting. An unmarried monster without a partner will reluctantly drink this when she needs immediate replenishment of mana, and as for monsters with companions, because they know the taste of the best and real thing, they are all the more unable to drink it. Even in Sabbath, there are many scenes where the recovery of mana is necessary along with magical research and experiments, so you often see the appearance of unmarried witches grumbling complaints of "I want an Onii-chan as soon as possible" every time they drink this.