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Traditional healers have used aromatic plants for millennia to alleviate numerous conditions, partly by helping people relax and partly through direct pharmacological actions.
For instance, aromatic herbs used to alleviate respiratory ailments include Eucalyptus citriodora and E. globulus, which contains the aromatic citronellal, peppermint (Mentha piperita), which contains menthol, and rosemary (Rosmarinus officinalis), which contains camphor.
“There is now extensive evidence from robust and controlled scientific and clinical studies that aromatics reduce the symptoms of upper respiratory tract infections [URTIs], including a blocked nose and cough, and prevent the sleep loss caused by these symptoms,” says Professor Andy Smith, School of Psychology, Cardiff University, and one of the authors of a recent review in Drugs in Context (2022; 11:2022-5-6), which was sponsored and co-authored by Procter and Gamble.
The review focuses on promoting restful and restorative sleep, which, the authors say, bolsters the body’s defences against URTIs. Sleep deprivation, for example, seems to impair lymphocyte and monocyte function – white blood cells that tackle URTIs.
Many URTI symptoms, especially nasal congestion and cough, disrupt sleep. Evidence is presented in the review that aromatic ointments containing menthol, eucalyptus oil and camphor applied before going to bed can reduce nocturnal cough, nasal congestion and sleep disturbances associated with the common cold.
“There are plausible underlying mechanisms that account for these benefits”
Numerous factors unrelated to nasal airway resistance – such as air temperature, stimulation of nasal cold receptors and the sensation of pressure – influence subjective sensations of nasal obstruction. “The efficacy of inhaling aromatics has been questioned because they do not change objective measures of nasal airway resistance,” Professor Smith says. “However, people inhaling aromatics feel that they have a clearer nose and this subjective experience underlies the increased wellbeing.”
Inhaled aromatics seem to work by modulating signalling pathways linked to cold receptors in the nasal airway, the authors argue. These cold receptors influence the sensation of nasal patency and patient comfort. On the one hand, sensing reduced or no nasal airflow gives rise to a sensation of stuffiness, even when nasal airflow resistance is normal. On the other hand, menthol creates a sensation of eased nasal breathing. Inhaling combined preparations of menthol, camphor and eucalyptus can relieve nasal congestion.
Aromatics’ effects probably arise, the authors suggest, because the active ingredients stimulate heat-sensitive, calcium-selective transient receptor potential (TRP) receptors. These act as ‘cold air’ receptors, which sense nasal airflow. Respiratory viruses may increase expression of the TRP receptors TRPM8, TRPV1 and TRPA1, which contributes to the sensation of congestion that is typical of URTIs.
Leading candidate
The authors suggest that TRPM8 is the leading candidate for mediating the sensation of nasal decongestion associated with menthol and citronellal in eucalyptus oil. Menthol, eucalyptus oil and camphor stimulate TRPV1 and TRPA1 receptors, which are expressed in the larynx and trigger cough.
Clinically, the review notes, menthol is antitussive, including in children with cold-associated nocturnal cough. The authors add that clinical trials suggest that proprietary preparations of aromatic ointments, containing menthol, eucalyptus oil and camphor alleviate some cold symptoms, including the sensation of nasal congestion, nocturnal cough and the associated sleep difficulties.
The authors suggest that pharmacists could recommend OTC ointments containing menthol, eucalyptus oil and camphor to relieve nocturnal cough, congestion and sleep difficulty caused by colds. While suitable for all patients with a cold, aromatics may be particularly useful in certain patients.
“Aromatics provide a good alternative to other compounds that may be contraindicated in vulnerable sub-groups, such as children, the elderly and those with other medical conditions,” Professor Smith adds.
Some people using ointments containing menthol, eucalyptus oil and camphor develop redness, irritation of the skin or eyes (the latter by inhalation) or allergic dermatitis. These effects are rare and when they arise are usually mild. People with a history of airway disease, pronounced hypersensitivity of the airways or asthma should be advised to be cautious and consider consulting a doctor before use.
Extensive evidence
“Our review shows that the evidence base for the efficacy of aromatics in the treatment of URTIs is extensive. These compounds can be used to treat a variety of symptoms, including a blocked nose, cough and sleep loss, and there are plausible underlying mechanisms that can account for these benefits,” Professor Smith concludes.
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