Estrogens all have a very strong first pass metabolism when taken orally. The liver breaks down most of the stuff, which not only reduces efficiency and is a strain on the liver, but also increases risk of blood clots (most of the panic about HRT risks comes from this). This is why sublingual is a good deal safer and more efficient and why other routes like transdermal or injections are generally preferrable, but oral is ok if you regularly get bloodwork done and don't have other blood clot risks (age, smoking, weight etc.)