Hops
Horehound
Horse Chestnut
Horsetail
Hyssop I
Iceland Moss
Irish Moss
Jamaican Dogwood
Juniper Berry
Kava Kava Root
Kelp: A reliable source of iodine and calcium. Kelp-derived iodine in prenatal vitamins is safe. The reason kelp is "not recommended for use during pregnancy" is because ingestion of *large amounts* of kelp during pregnancy may result in congenital hypothyroid of the fetus, possibly resulting in mental retardation and delayed central nervous system development. Ingestion of kelp during pregnancy has also been associated with congenital cataracts.
Kola Nut
Lemon Balm: Not enough clinical information is available on how lemon balm might affect a developing fetus, an infant, or a small child. Therefore, its internal use is not recommended during pregnancy, while breast-feeding, or during early childhood.
Lemon Grass: A culinary herb and a common ingredient of "Mothers Milk Tea" found in many herb and health food stores. The American Herbal Products Association Botanical Safety Index says that lemongrass is not to be used during pregnancy due to uterine and menstrual flow stimulation. There is no apparent danger for use in blended teas, occasional culinary use, or topical applications. I could not find any evidence of harmful effects from the use of lemongrass by breastfeeding mothers.
Licorice Root
Lobelia
Lotus Leaf
Lovage
Lungwort
Lycii Berry