The Antibiotics are a drug Accessory.
When equipping this to any girl with one or more curable STD's, a single STD will be healed after 3 days.
This is a temporary item. Once the 3 days are passed, the item will disappear leaving the slot open for other items, but the STD will remained cured.
As The Player has a choice in giving the girls Condoms, which will guarantee safety during escorts, the only reason Antibiotics will be needed is if The Player is a risk taker or misdrags a girl onto the Sleazy Motel. In most cases, Antibiotics will take care of an STD, but if the STD is severe enough to be permanent and incurable, it will require Steroids to null the effects for three days while the Antibiotics become useless unless another girl falls ill due to the same mistake.