Factor V (FV) deficiency, also known as parahemophilia, is caused by mutations in the FV gene (or related genes) or is acquired in the setting of rheumatology disorders, malignancies, antibiotic use, or topical bovine thrombin. Many patients with factor V levels of less than 1% have only mild to moderate bleeding.

The inherited form is autosomal recessive and can result in mild, moderate, or severe bleeding. Patients are usually homozygous or compound heterozygous for mutations in the FV gene (F5). Heterozygous carriers have about half-normal levels of FV and are usually asymptomatic. The estimated prevalence of FV deficiency is 1:1 million.

A procoagulant state has also been described in association with FV deficiency. This may be due to inherited mutations of both FV deficiency and FV Leiden.