Laboratory research and limited trials suggest that cloves and their active compound, eugenol, may have properties effective against some viruses and inflammation associated with respiratory diseases. However, these findings have not yet reached the level of clinical evidence required to establish cloves as a treatment for infections in humans.
The composition of cloves and their traditional medicinal uses
Controlled human studies are still needed before findings relating to Covid-19 or influenza, or the use of eugenol to limit the airborne transmission of viruses, can be generalised. Cloves, whose scientific name is Syzygium aromaticum, have been used for centuries as a spice in food preservation, owing to their antioxidant and antimicrobial properties.
Multiple studies have pointed to their potential ability to combat viruses, bacteria and fungi, as well as to anaesthetic and antiseptic properties associated with their use in various healthcare applications. Cloves are native to the Maluku Islands in eastern Indonesia, while their trees are now cultivated in several countries, including India, Madagascar, Sri Lanka and Malaysia.
Cloves contain vitamins, minerals and phenolic compounds, as well as eugenol and gallic acid, which accounts for 89% of the essential oil found in clove flower buds, according to data on their composition. These compounds are biologically active, giving cloves potential medicinal value alongside their culinary use.
Their use in traditional medicine also dates back more than 2,000 years, including in Chinese folk medicine and the Indian traditional medicine known as Ayurveda. Health uses of cloves have taken various forms, including essential oil, dried buds and powder. The oil has been used to address gum inflammation, burns, some digestive disorders, joint pain and spasms.
Some studies have found potential anti-tumour and anti-inflammatory properties, as well as properties that inhibit the formation of new blood vessels, alongside an analgesic effect that has contributed to their widespread use in relieving toothache and headaches. Regarding fungi, a study by Moroccan researchers found eugenol effective against vaginal candidiasis in laboratory rats.
Clove oil has also been attributed with antioxidant activity, a property relevant to oxidative stress, which can affect brain cells and cognitive functions. Dr Sumita Halder and colleagues at the University College of Medical Sciences in India recorded improvements in short- and long-term memory in mice treated with clove oil.
The researchers suggested that this improvement was linked to the oil's potential ability to reduce oxidative stress, but findings from animals alone do not constitute clinical evidence of the same effect in humans. Cloves also have anticoagulant and anti-inflammatory properties, with a potential ability to enhance immunity.
Interest in cloves rises during the respiratory infection season
These characteristics have increased interest in cloves in the context of viral infections affecting the respiratory system, particularly during the Covid-19 pandemic caused by the SARS-CoV-2 virus, which began in early 2020. Interest in natural remedies resurfaces with the arrival of autumn and winter and the increased spread of viral respiratory diseases among people of different age groups.
Most interest in the antiviral properties is centred on eugenol, which is present in high concentrations in cloves. The proposed mechanism suggests that the compound binds to viral antigens, proteins or glycoproteins found on the surface of the viral cell, potentially inhibiting the virus's growth and spread within the host. Eugenol has previously been shown to inhibit the growth of herpes simplex virus.
Eugenol studies reveal activity against some viruses
In 2007, researchers Trajolbua and Jatisatienr from Chiang Mai University in Thailand published a study showing eugenol's effectiveness against some viruses.
As the novel coronavirus spread, some infected people, particularly those with weakened immunity, began combining medicines and herbs in an effort to benefit from their potential antiviral and anti-inflammatory properties and support the body's response to infection.
Reports that emerged after the pandemic said hospitals in China's Wuhan province, where the virus's spread began, moved quickly at the start of the outbreak to treat infected people with traditional Chinese medicine formulas, recording effectiveness estimated at about 90% against the virus. Such figures remain linked to the nature of the reports and the assessment methods used, and are not sufficient on their own to prove the effectiveness of cloves specifically.
Covid-19 trial involved only 10 patients
In June 2020, Professor Jean-Noël Mbuto of the University of Kinshasa in the Democratic Republic of the Congo published findings on five medicinal plants, including dried clove buds, as part of an attempt to treat, prevent and control Covid-19 cases at a time before vaccines became available.
Mbuto's trial involved a small sample of 10 patients infected with Covid-19. Participants were asked to inhale vapours from a boiled infusion prepared according to specified steps and criteria, containing cloves, ginger, mint, lemon grass leaves and eucalyptus. The patients also drank an infusion of cloves and other herbs in the morning and evening.
The trial found that the mixture produced what was described as a good response against the infection by stimulating the body's immunity and directly easing some symptoms, including headaches. However, the small sample size and absence of a control group prevented a more accurate comparison of the results or the separation of the herbal mixture's effect from other factors.
Four participants also took azithromycin, an antibiotic, alongside the herbs. This factor affects the ability to assess the effectiveness of the plants independently of the drug and, together with the absence of a control group, limits the possibility of generalising the trial's findings.
Regarding the prevention of bacterial complications, Tunisian researchers outlined the ability of clove essential oil to combat bacteria and inhibit their growth in a review published in 2007 in Phytotherapy Research. On the basis of these properties, Dr Muslatani and Dr Rahimi recommended in 2020 using clove oil to prevent secondary bacterial infections that may affect Covid-19 patients as a result of weakened immunity.
The first laboratory trial examining eugenol's effectiveness against Covid-19 infection was published in 2021 in the Journal of Neuroimmune Pharmacology. The results showed that orally administered eugenol from cloves helped ease lung irritation and inflammation, improve heart function, increase physical activity and reduce fever in mice infected with the disease.
Nevertheless, findings based on cells or animals still need to be confirmed through controlled clinical trials in humans with adequate sample sizes, so that effectiveness, dosage and safety can be determined precisely. The studies cited therefore do not yet provide sufficient grounds for establishing cloves as a proven treatment for Covid-19 or other respiratory diseases.
At the level of popular use, people in tropical parts of Asia have long relied on cloves to ease some respiratory problems. Practices have included drinking a boiled infusion and inhaling its volatile aromatic vapours to help with coughs, colds, respiratory attacks and bronchitis, as well as chewing cloves to relieve sore throats and irritation of the pharynx.
Clove oil has been used as an expectorant to help expel phlegm in cases of upper respiratory tract infection, based on its antiviral and antibacterial properties. Some studies have also indicated that mixing clove oil with honey may help ease chronic coughs, breathing difficulties and heart palpitations, but this does not make these uses a proven alternative to medical treatment.
Researchers have also tested the possibility of using eugenol to disinfect the air or reduce its viral load, potentially limiting transmission between people. The idea is based on the compound's volatile nature, which helps it spread quickly and may enable it to affect viruses carried between airborne particles.
In 2016, researchers from Canada and France indicated that spraying products containing eugenol could help reduce the risk of viruses spreading through airborne droplets. On the basis of the same properties, two researchers from Japan's Asahi University proposed in 2024 that eugenol might limit the spread of viruses, particularly SARS-CoV-2, inside dental clinics.
The researchers linked this hypothesis to the possibility of eugenol leaking into the air during molar drilling procedures, or being released from fillings and adhesives used in various dental treatments, which could explain part of the distinctive smell of dental clinics. However, this proposal remains an initial hypothesis requiring further research for verification and scientific proof.
Cloves are generally safe when used in moderate amounts, and the US Food and Drug Administration has listed them among the ingredients permitted for use as a safe dietary supplement.
However, excessive consumption, particularly in large amounts, may be associated with seizures, liver damage and blood-clotting disorders, making it necessary to adhere to moderate quantities in line with recommendations from specialists and experts.
