Leeches only feed about once every six months, this is about how long the blood meal takes to be fully digested. Certain bacteria keep the blood from decaying during the long digestion period. H. medicinalis may even go longer than six months without food by digesting its own tissues. It also remains fertile over a period of years,unlike most other leech species. The act of copulation takes place on land, where one leech attaches ventrally to one another by means of a mucus secretion.
All leeches are hermaphroditic and fertilization is internal.
Sperm is injected into the vagina by an extendable copulatory organ. A cocoon is formed around the clitellum and slips off the anterior section of the leech. The whole egg sac is laid in damp soil usually just above the shoreline. Hirudo medicinalis moves in water by contraction of the longitudinal muscles of the body in a wave-like motion which propels it forward in the water. They attach themselves to the substrate alternately by their anterior and posterior suckers.