The Science of Rosemary and Blood Health
Rosemary (Rosmarinus officinalis L.) is more than a fragrant culinary herb; it's a plant with significant pharmacological properties, largely due to its rich content of antioxidants like carnosic acid and rosmarinic acid [1.2.1, 1.3.8]. These compounds are at the heart of rosemary's effects on the body's vascular system. Understanding what rosemary does to the blood requires a look at its influence on circulation, blood pressure, blood sugar, and its potential for critical drug interactions.
Improved Circulation and Blood Flow
Rosemary is known to improve blood circulation [1.2.2]. This effect is partly due to its anti-inflammatory properties and its ability to help blood vessels dilate, or widen [1.2.1]. The active compounds in rosemary can relax blood vessels by activating specific potassium channels (KCNQ4 and KCNQ5), which hyperpolarizes the smooth muscle cells in the vessel walls [1.5.1]. Improved circulation means better blood flow to muscles and organs. This is why rosemary oil is sometimes used topically to soothe aching muscles, as it helps deliver more oxygenated blood to the inflamed tissues [1.2.1, 1.2.8]. This vasodilatory effect may also support cognitive function by increasing blood flow to the brain [1.2.1].
The Dual Effect on Blood Pressure
Rosemary's impact on blood pressure is complex and can be contradictory. Some evidence suggests it can be beneficial for individuals with hypertension. Studies have found that rosemary acts as an ACE (angiotensin-converting enzyme) inhibitor [1.2.1]. ACE inhibitors work by blocking the formation of angiotensin II, a hormone that constricts blood vessels and raises blood pressure [1.2.1]. A 2025 study showed that daily consumption of rosemary infusion over 45 days significantly reduced both systolic and diastolic blood pressure in patients with grade 1 hypertension, but had no significant effect on healthy individuals with normal blood pressure [1.2.4].
Conversely, some sources caution that large, medicinal doses of rosemary might increase blood pressure [1.2.8, 1.6.3]. Early research has also suggested it can be used to increase blood pressure in those with hypotension (low blood pressure) [1.2.3, 1.2.7]. This paradoxical effect highlights the importance of dosage and individual health conditions. While regular culinary or tea amounts may be beneficial for hypertension, concentrated extracts or high doses could have an opposite, stimulatory effect [1.6.2].
Aiding in Blood Sugar Regulation
Multiple studies indicate that rosemary can help lower blood sugar levels, making it a subject of interest for managing hyperglycemia and type 2 diabetes [1.3.4]. The active compounds, carnosic acid and rosmarinic acid, have been shown to exert insulin-like effects [1.3.4]. They may help lower blood sugar by increasing the absorption of glucose into muscle cells [1.3.4, 1.3.8].
Another mechanism is the inhibition of enzymes like alpha-glucosidase, which are responsible for breaking down carbohydrates in the intestine. By slowing this process, rosemary can lead to a slower and lower rise in blood glucose after meals [1.2.1, 1.3.2]. Studies in both animals and humans have demonstrated that rosemary powder or extract can significantly reduce fasting blood glucose and HbA1c levels [1.3.2, 1.3.7].
Antioxidant Protection for Blood Cells
The powerful antioxidant properties of rosemary are central to its benefits for blood health [1.2.1]. Oxidative stress, caused by an imbalance of free radicals and antioxidants, can damage cells, including red blood cells and the delicate lining of blood vessels (the endothelium) [1.7.2]. The compounds in rosemary, particularly carnosic and rosmarinic acid, are effective at scavenging these free radicals [1.5.5]. By doing so, they protect red blood cells from oxidative damage and hemolysis (the rupture of red blood cells) [1.7.2, 1.7.4]. This protective effect helps maintain the integrity and function of blood cells and vessels [1.7.1].
Comparison of Rosemary Forms and Blood Effects
The way you consume rosemary can influence its effects on the blood.
| Feature | Rosemary Tea | Rosemary Extract | Rosemary Essential Oil |
|---|---|---|---|
| Primary Application | General wellness, mild digestive aid | Concentrated dose for targeted effects [1.3.8] | Aromatherapy, topical application [1.2.8] |
| Blood Sugar Effect | Mild to moderate reduction [1.3.1] | Moderate to strong reduction [1.3.2] | Negligible (if not ingested) |
| Blood Pressure Effect | May lower in hypertensive individuals [1.2.4] | Dose-dependent; can lower or raise [1.2.8] | Can lower via stress reduction (inhalation) [1.2.1] |
| Circulation Effect | Mild systemic improvement [1.2.2] | Moderate systemic improvement | Strong local improvement (topical use) [1.2.8] |
| Bleeding Risk | Low in culinary amounts [1.4.2] | Moderate to High [1.6.3] | Low (unless ingested) [1.6.1] |
Critical Safety: Interactions with Blood-Thinning Medications
One of the most significant pharmacological effects of rosemary is its potential to interfere with blood clotting [1.4.1]. Rosemary has anti-thrombotic and antiplatelet actions, meaning it can slow the blood's ability to clot [1.2.1, 1.4.2]. This effect is similar to that of aspirin [1.6.1].
Because of this property, taking rosemary in medicinal amounts alongside anticoagulant (e.g., warfarin, heparin) or antiplatelet (e.g., clopidogrel, aspirin) drugs can substantially increase the risk of bruising and bleeding [1.4.3, 1.4.4]. Individuals on these medications should only consume rosemary in normal culinary amounts and must consult their healthcare provider before using rosemary supplements or extracts [1.4.2, 1.4.5]. It is also contraindicated for people with bleeding disorders [1.6.1].
Conclusion: A Potent Herb Demanding Respect
Rosemary clearly has profound and varied effects on the blood. Its antioxidant and anti-inflammatory compounds can improve circulation, protect blood cells, help manage blood sugar, and potentially lower high blood pressure [1.2.1, 1.3.4]. However, its potency is also the source of its risks. The paradoxical effect on blood pressure and, most importantly, the significant interaction with blood-thinning medications, require caution [1.2.8, 1.4.1]. Large quantities can also be toxic, potentially causing issues like kidney irritation or vomiting [1.6.1, 1.6.4]. Therefore, while rosemary is a beneficial herb, its use for medicinal purposes should always be discussed with a healthcare professional, especially for individuals with pre-existing conditions or those taking other medications [1.2.8].
For more detailed information, you can review this article from the National Institutes of Health: https://pmc.ncbi.nlm.nih.gov/articles/PMC5622728/
