Celery seed has long history of medicinal use for a variety of health complaints including gout, arthritis and high blood pressure. It contains a multitude of compounds that affect inflammatory processes in the body as well as other regulatory functions.
Apigenin is a compound contained in celery seed that is thought to contribute to its beneficial effects on blood pressure. It is thought to help blood vessels relax and expand which causes a reduction in intra-arterial pressure. Many other compounds in celery might also contribute to its blood pressure lowering capability. It has long been thought to be a mild diuretic which can be a tool used to lower blood pressure.
It is not widely studied for this use but lots of anecdotal reports suggest it can be helpful for regulating blood pressure.
Generally speaking, finding a standardized extract is your best chance of having a positive outcome. No official dosing schedule has been established but doses ranging from 500mg-4000mg of a standardized extract daily are generally recognized as safe. If you have pre-existing kidney disease it is not advisable to take celery seed extract. As with most herbal preparations, it is advisable to take between meals to maximize absorption.