I can't believe so many people don't get the joke.
The alcohol in question in Yomeishu, a herbal liqueur that is supposedly as healthy as drinking herbal soup. If you have consumed authentic Chinese herbal brews, herbal medicines or herbal soups, you should know what to expect of Yomeishu. Basically, it tastes like utter shit.
All four love interests gave Reiji a bottle of that stuff each last year on Valentine's Day out of concern for his health, which he totally did not enjoy. And from the looks of it, history is about to repeat this year, except with candies and sweets stuffed with Yomeishu.
If it tastes so bad, putting it in chocolates seems like it would be a prank gift rather than a serious one.
Doesn't look like a prank because they genuinely believe that it's for the sake of his health. Just look at post #3019882.
Try imagining something as sweet as chocolate mixed with the bitter, earthy taste of ginseng and the tangy flavor of turmeric. The last two is what Yomeishu tastes like (to me, YMMV)
I had the misfortune of tasting Yomeishu when I was 10. My parents refused to let me try any alcoholic drinks (well duh, I was underage), but they were willing to close an eye for health liqueurs like D.O.M Bénédictine and Yomeishu.