Some people with severe haemophilia develop an antibody or inhibitor to factor treatment. The effect of this is that when the factor is given for a bleed there is no clinical response because the activity is immediately neutralised. Often the first sign that a person has inhibitors is that his treatment does not appear to be working.

Inhibitors to Factor VIII occur in about twenty percent of individuals with Factor VIII deficiency. The inhibitors can be high titre or low titre. High titre inhibitors refer to a higher level of inhibitors which is more clinically significant and more difficult to eradicate. Low titre inhibitors are often transient and the person can still be treated with factor concentrates at higher doses. If a child develops a high titre Factor VIII inhibitor, the optimum therapy is immune tolerance therapy where the child is treated with very high doses of Factor VIII for an average period of about one year, in an attempt to eradicate the inhibitors. Treatment for Factor VIII inhibitors can be with a number of agents including a recombinant Factor VIIa (Novo Seven) or an alternative product known as FEIBA.
Inhibitors in patients with Factor IX deficiency are extremely rare and are more difficult to treat. In general, children and adults with haemophilia are screened for inhibitors at least once or twice per annum.