Function:
Rosemary (Rosmarinus officinalis) is widely used as a spice when cooking, especially in Mediterranean dishes. It is also used for its fragrance in soaps and other cosmetics. Traditionally, rosemary has been used medicinally to improve memory, relieve muscle pain and spasm, stimulate hair growth, and support the circulatory and nervous systems. It is also believed to increase menstrual flow, act as an abortifacient (causing miscarriage), increase urine flow, and treat indigestion. Almost none of these uses have been studied scientifically in humans. However, one study in humans found that long term daily intake of rosemary prevents thrombosis.
In the lab, rosemary has been shown to have antioxidant properties. Antioxidants can neutralize harmful particles in the body known as free radicals, which damage cell membranes, tamper with DNA, and even cause cell death. Also in the lab, rosemary oil appears to have antimicrobial properties (killing some bacteria and fungi in test tubes). It isn't known whether rosemary would have the same effect in humans.
Indigestion:
Rosemary leaf is used in Europe for indigestion (dyspepsia) and is approved by the German Commission E, which examines the safety and efficacy of herbs.
Muscle and Joint Pain:
Applied topically (to the skin), rosemary oil is sometimes used to treat muscle pain and arthritis and to improve circulation. It is approved by the German Commission E for this purpose. However, there is no scientific evidence that it works.
Alopecia:
Historically, rosemary has been used to stimulate hair growth. Rosemary was used in one study of 84 people with alopecia areata (a disease in which hair falls out, generally in patches). Those who massaged their scalps with rosemary and other essential oils (including lavender, thyme, and cedarwood) every day for 7 months experienced significant hair regrowth compared to those who massaged their scalps without the essential oils. But the study was not well designed, and it is impossible to say whether rosemary caused the hair growth.
Neutralize Foodborne Pathogens:
Several studies show that rosemary inhibits foodborne pathogens like Listeria monocytogenes, B. cereus, and S. aureus.
Improve Memory or Concentration:
Rosemary is often used in aromatherapy to increase concentration and memory, and to relieve stress. One study suggests that rosemary, combined with other pleasant smelling oils, may lower cortisol levels and help reduce anxiety. Another study found that the use of lavender and rosemary essential oil sachets reduced test taking stress in graduate nursing students.