Delayed infection, occurring a few days after leech therapy has been reported and caution is advised.

Optimal care is recommended when applying leeches because their use can be complicated by serious bacterial infections.

Leech therapy may interact with antibiotic therapy such as trimethoprim and sulfamethoxazole (Cotrim forte) and may precipitate allergic reactions. In one case study, Beer et al. reported an allergic reaction that developed after four days of leech therapy and antibiotic use, which was reversed using systemic doses of glucocorticoids. This case was not considered typical.

The benefits of leech therapy are due, in large part, to the anticoagulant effects ("blood thinning"), vasodilatory effects, and anesthetic effects of the biochemicals contained in leech saliva, as well as the physical effects of blood letting (phlebotomy). Hirudin, a potent anticoagulant in leech saliva, inhibits the conversion of fibrinogen to fibrin, preventing blood from clotting. Indeed, a wound may continue to bleed for many hours after the leech has already detached due to the anticoagulant effect of hirudin. Many recently developed prescription drugs used for similar conditions were designed based on the mechanism of action of leeches.

Leeches are used to assist in the reattachment of severed body parts such as fingers, hands, toes, legs, ears and noses. Leeches are used to help with venous insufficiency when there is sufficient arterial flow when reattaching severed body parts.

Today leeches are also used to drain blood from swollen faces, limbs and digits (fingers and toes) after reconstructive surgery. They are especially useful when reattaching small body parts that have many small blood vessels. Leeches are used to prevent the clotting of blood in these small veins. Leeches are gently placed in the area needed by a qualified healthcare provider, and allowed to attach and engorge for the next 6-12 hours, after which they will release. The entire course of treatment may require one to 6 treatments or more, depending upon the goals and rate of response.

The leech will withdraw approximately 5mL (one teaspoon) of blood. Further therapeutic benefit of leech therapy comes after the leech is removed, during which up to 50mL of blood will continue to ooze, for up to 48 hours. More leeches attached to the site will result in the removal of more blood. After 3-7 days, the veins have usually reconnected themselves such that the blood is no longer pooling in the limb. Normal color and pressure should return to the area, as arterial blood circulates easily in the damaged zone. By that time, the wound will be able to heal, without further phlebotomy (removing blood).

Evidence: In 1998, Bapat et al. investigated the use of medicinal leeches in the treatment of complicated varicose veins and found that medicinal leeches aided ulcer healing and could be used as an adjunct in the management of complicated varicose veins due to their blood sucking ability. In a randomized controlled trial in 2003, Michalsen et al. found that medicinal leech therapy was effective in relieving symptoms in patients with osteoarthritis of the knee. Based on clinical use, the leech may also be of value in those performing microsurgery who are faced with the difficulties of reattaching minute veins.

The application of leeches to the patient is relatively simple, but does require application by a qualified healthcare professional. As few as one, or as many as six or more leeches may be required for a wound, depending upon its size and its clinical response. The greatest number of leeches should be applied by the healthcare provider to the area of maximal venous congestion.

The patient's skin is typically cleaned thoroughly with soap and water, and then rinsed with distilled, non-chlorinated water. A gauze barrier around the area intended for the leech is used to prevent the leech from wandering away from the site where its attachment is desired.

Once the leech is attached, it will likely remain safely in place until fully distended. The site should be checked continuously to insure that the leech has not moved. The leech will let go of the patient (host) when it is finished (usually within an hour).

If the leech is reluctant to bite, it might be necessary to entice it with a tiny droplet of blood, drawn from the wound site with a needle prick.