Before applying leeches the skin should be thoroughly cleansed. If the leech shows no inclination to take hold a little sweetened water or milk rubbed on the part will usually obviate the difficulty. If it be desirable to accurately place the leech, a glass tube three or four inches in length, open at both ends, and of somewhat greater diameter than the leech, known as a "leech glass," is used. Into this the leech is dropped and the end of the tube placed over the point of application.

A leech will not draw more blood than is desirable. For that reason they are left on until they let go of their own accord, and the amount of blood to be removed is governed by the number of leeches put on at one time, it being usual to put on three or four. If, for any reason, one desires to remove the leech, it can be made to let go by dropping on it a bit of salt. It should not be pulled off.

The Swedish leech will draw about 20 c. c. of blood; the American leech about 10 c. c. The so-called "artificial leech" is a wet cup.