Hirudin is the anticoagulative product of the salivary glands of the medical leech Hirudo medicinalis. It is characterized by a direct, bifunctional inhibition mechanism and a high, exclusive specificity and a strong ability to bind to thrombin (tight binding). Further characteristics are the organic-chemical structure of hirudin (peptide), which allows only parenteral administration; the missing metabolism in the organism; and the exclusive glomerular filtration of hirudin in kidneys as the effective elimination mechanism. Recombinant hirudin (r-hirudin) is a product of genetic engineering that is identical to the hirudin found in nature and has the same biochemical and pharmacological characteristics as natural hirudin.