Melaleuca cajuputi

Minyak kayu putih is arguably the most domesticated essential oil in the
Malay world: distilled from the leaves and twigs of the gelam tree, kept in
the family cupboard, rubbed on chests and temples, held under a blocked
nose. The Isah 2023 scoping review documents the tree's traditional uses
across the region alongside its studied biological activities [1]. The oil
entered global commerce centuries ago under the name cajuput — a direct
borrowing of "kayu putih", the white-barked tree it comes from.

The oil's character comes from 1,8-cineole. Indonesian plantation oils
assay at roughly 49–55% cineole with α-terpineol, limonene and
β-caryophyllene alongside [2], and a metabolomic survey of thirteen
Indonesian cajuput oils confirmed the same principal constituents with
origin-dependent variation [3]. The variation is not academic: a Cuban
study of the same species group found a viridiflorol-dominant chemotype at
only 21% cineole [4]. Cineole content cannot be assumed from the species
name — it must be specified per batch.

The clinical evidence usually offered for cineole-rich vapour products
belongs to a different plant and a different route. Kehrl and colleagues
(2004) ran a double-blind, placebo-controlled trial of purified cineole —
eucalyptus-derived, in oral capsules — in 152 patients with acute
rhinosinusitis and reported meaningful symptom improvement [5]. Worth and
colleagues (2009) found oral cineole at 200 mg three times daily reduced
exacerbations in COPD in a placebo-controlled trial of 242 patients [6].
Both are trials of purified 1,8-cineole taken orally, and neither tests
cajuput oil or a vapour route.

Cajuput-specific research is at the laboratory bench: Al-Abd and
colleagues (2015) characterised the extract's activity against
Staphylococcus aureus and other organisms in vitro [7]. That is the
honest position: a household practice of long standing, a well-studied
principal constituent, and no clinical trial of the oil itself in its
traditional use. Grade C.

One human trial of the oil itself has now been published. An Indonesian
randomised controlled trial in 127 patients with mild-to-moderate COVID-19
used cajuput oil inhalation as an adjuvant to standard therapy and
reported a shorter hospital stay [10]. It is a single trial, in one
country, for an indication far from traditional household use — but it is
the first clinical evidence for cajuput oil rather than for eucalyptus
cineole, and it is why the ladder now reaches the clinical rung. The
grade stays at C: one trial in an unrelated indication does not establish
the traditional use.

A recent Malaysian review of the species is explicit that safety studies
and clinical trials remain outstanding [11].

The hazard literature is unambiguous where it matters most: small ingested
volumes of cineole-rich oils have caused central nervous system depression
and seizures in infants and young children, documented by Tibballs (1995)
for eucalyptus oil [8] with modern case reports continuing to accrue [9].
The same hazard transfers directly to minyak kayu putih in homes with
small children: keep the bottle out of reach, never administer orally,
and do not apply near the face of an infant. On adult skin the diluted
oil is generally well tolerated; the neat oil can irritate.

A regulatory-grade assessment now exists: the European Food Safety
Authority's 2024 opinion on cajuput oil sets out a compositional
specification and exposure assessment for the oil as such [12]. It
concerns animal feed use rather than human vapour, but it is the most
authoritative compositional and safety document available for this
specific oil, and it reinforces the point above about specifying batch
cineole content rather than assuming it.