Those at risk for birth control - specifically speaking of hormonal birth control - are those that have a hypercoagulable state. That is, they personally have had a history of a blood clot formation. In particular, we're speaking of deep venous thrombosis and/or pulmonary embolus. That is: blood clots in your extremities, particularly your legs and/or a blood clot in your lungs. Also, people that have hormonal cancers - in particular, some breast cancers and endometrial cancers - typically are not candidates for hormonal birth control.