Oral contraceptives, venous thrombosis, and varicose veins. Royal College of General Practitioners' Oral Contraception Study.

To refine 1974 results on venous thrombosis and oral contraceptives (OCs) from a prospective study begun in 1968 comparing 23,000 women using OCs with 23,000 controls, women with known preexisting conditions relevant to thrombosis were excluded. Rates in each contraceptive category (users, ex-users, and controls) were indirectly standardized for age, parity, cigarette consumption, and social class. The relative risk (users to nonusers) of deep vein thrombosis was 4.17; for superficial vein thrombosis, it was 2.41. For other thrombosis and pulmonary embolism, trends were similar but the differences were not significant. Rates of ex-users did not differ significantly from the controls, implying no persistent risk after OC use ends. Superficial thrombosis correlated significantly with age, parity, and estrogen dose and progesterone content of OCs, but deep vein thrombosis did not. The risk of superficial thrombosis was significantly increased in users and nonusers with severe varicose veins, and increased with the severity; no significant relationship was found with deep vein thrombosis.