Description: <p><b>Geraniol</b> — an acyclic monoterpene alcohol and fragrance compound found in citronella, palmarosa, rose, lemongrass, rose-geranium, and several other essential oils. It is formally classified as a plant-derived monoterpenoid natural product; Citronella oil is not equivalent to geraniol: it is a variable multi-component essential oil distilled primarily from <i>Cymbopogon winterianus</i> or <i>Cymbopogon nardus</i>, with citronellal, geraniol, citronellol, geranyl acetate, limonene, and other terpenes as principal constituents. </p>
<p><b>Primary mechanisms (ranked):</b></p>
<ol>
<li>Induction of intrinsic and caspase-dependent apoptosis through mitochondrial dysfunction, altered BAX/BCL-2 balance, cytochrome-c release, and caspase activation.</li>
<li>Suppression of PI3K/AKT/mTOR survival and growth signalling.</li>
<li>Disruption of mevalonate and lipid metabolism, including inhibition of HMG-CoA reductase activity and reduced availability of intermediates required for membrane synthesis and protein prenylation.</li>
<li>Suppression of NF-κB, inflammatory cytokine, MAPK, and JAK/STAT3 signalling in responsive cancer models.</li>
<li>Cell-cycle arrest and inhibition of DNA synthesis, proliferation, migration, invasion, and epithelial–mesenchymal transition.</li>
<li>Chemosensitization, particularly enhancement of 5-fluorouracil activity in colorectal-cancer models.</li>
<li>Redox modulation, with pro-oxidant mitochondrial stress reported in some cancer models but antioxidant and NRF2-associated cytoprotection reported in non-cancer and injury models; direction is strongly context-dependent.</li>
</ol>
<p><b>Bioavailability / PK relevance:</b> Geraniol is lipophilic and can be absorbed after oral administration, but it is rapidly distributed and extensively converted to geranic acid, dihydrogeranic acid, glucuronide conjugates, and other metabolites. Rat studies indicate a short blood half-life and large formulation-dependent differences in oral bioavailability. Recent mouse studies likewise show rapid metabolism, so free-geraniol exposure is transient. Emulsions, lipid carriers, nanoformulations, and encapsulation may increase exposure, but these delivery systems do not establish clinical anticancer efficacy. Citronella-oil composition and exposure vary substantially with species, chemotype, cultivation, storage, and formulation.</p>
<p><b>In-vitro vs systemic exposure relevance:</b> Many anticancer experiments use geraniol concentrations in the tens to hundreds of micromolar range, and some use still higher levels. These sustained concentrations may exceed free systemic concentrations achievable through ordinary dietary or flavouring exposure because geraniol is rapidly metabolized and cleared. Direct comparison is difficult because human plasma PK data for therapeutic dosing are limited. Cytotoxic findings from undiluted or concentrated citronella oil should not be attributed solely to geraniol because citronellal, citronellol, methyl isoeugenol, limonene, and minor constituents may contribute independently or interact.</p>
<p><b>Clinical evidence status:</b> Preclinical. Evidence consists primarily of cancer-cell studies, chemically induced animal-tumour models, and xenograft studies. Geraniol has shown enhancement of 5-fluorouracil in colorectal-cancer models, but there are no established randomized controlled trials demonstrating that isolated oral or systemic geraniol treats cancer. A clinical study of a multi-ingredient topical essential-oil formulation for HPV-related disease cannot establish geraniol-specific efficacy. Neither geraniol nor citronella oil is an approved anticancer treatment or validated oncology adjunct.</p>
<p><b>Safety / regulatory relevance:</b> Geraniol is widely used as a flavouring and fragrance ingredient, while citronella oil is also used as a flavouring and insect-repellent ingredient. Food-use safety evaluations do not establish safety at pharmacological anticancer doses. Geraniol is a recognized fragrance allergen and can cause allergic contact dermatitis, particularly after oxidation. Concentrated citronella oil can irritate skin, eyes, mucosa, and the gastrointestinal tract and should not be treated as interchangeable with food-grade geraniol. Citronella oil also contains composition-dependent constituents, including methyl isoeugenol in some preparations, that require separate toxicological consideration.</p>
<h3>Geraniol Cancer Mechanisms</h3>
<table>
<thead>
<tr>
<th>Rank</th>
<th>Pathway / Axis</th>
<th>Cancer Cells</th>
<th>Normal Cells</th>
<th>Primary Effect</th>
<th>Notes / Interpretation</th>
</tr>
</thead>
<tbody>
<tr>
<td>1</td>
<td>Mitochondrial apoptosis</td>
<td>↑ BAX/BCL-2 ratio; ↑ cytochrome-c release; ↑ caspase-9 and caspase-3; ↓ mitochondrial membrane potential</td>
<td>↔ or ↓ apoptotic injury in some oxidative-stress models (context-dependent)</td>
<td>Apoptosis and reduced tumour-cell survival</td>
<td>One of the most consistently reported endpoints, but effective concentrations and upstream triggers vary by cell line.</td>
</tr>
<tr>
<td>2</td>
<td>PI3K AKT mTOR signalling</td>
<td>↓ PI3K; ↓ phosphorylated AKT; ↓ mTOR and downstream survival signalling</td>
<td>↔ or ↑ AKT-mediated survival in selected injury models (context-dependent)</td>
<td>Reduced proliferation, survival, protein synthesis, and treatment resistance</td>
<td>Observed in oral, nasopharyngeal, prostate, and other experimental cancer systems; direct molecular binding has not been established consistently.</td>
</tr>
<tr>
<td>3</td>
<td>Mevalonate and lipid metabolism</td>
<td>↓ HMG-CoA reductase activity; ↓ mevalonate-pathway flux; altered fatty-acid and phospholipid metabolism</td>
<td>↓ cholesterol synthesis (dose-dependent)</td>
<td>Reduced membrane synthesis, proliferation, and potentially protein prenylation</td>
<td>Mechanistically important in hepatocarcinoma and chemically induced colorectal-tumour models. Rescue by mevalonate has not been demonstrated uniformly across models.</td>
</tr>
<tr>
<td>4</td>
<td>NF-κB inflammatory survival signalling</td>
<td>↓ NF-κB activation; ↓ inflammatory and anti-apoptotic signalling</td>
<td>↓ NF-κB-driven inflammation in several non-cancer models</td>
<td>Reduced survival, inflammation, invasion, and apoptosis resistance</td>
<td>NF-κB modulation may be downstream of AKT inhibition or redox changes rather than a single direct target.</td>
</tr>
<tr>
<td>5</td>
<td>JAK STAT3 signalling</td>
<td>↓ STAT3 activation; ↓ survival and proliferation signals</td>
<td>Insufficient evidence</td>
<td>Apoptosis and suppression of tumour-promoting transcription</td>
<td>Reported in selected thyroid and other cancer-cell models; breadth across tumour types remains uncertain.</td>
</tr>
<tr>
<td>6</td>
<td>MAPK stress and proliferation signalling</td>
<td>↓ or altered ERK, JNK, and p38 signalling (model-dependent)</td>
<td>↔ or protective modulation (context-dependent)</td>
<td>Cell-cycle arrest, stress signalling, and apoptosis</td>
<td>The direction differs by cell type, concentration, and treatment duration; the MAPK family should not be represented as uniformly inhibited.</td>
</tr>
<tr>
<td>7</td>
<td>Cell cycle and DNA synthesis</td>
<td>↓ DNA synthesis; ↓ cyclin-associated progression; ↑ cell-cycle arrest</td>
<td>Insufficient evidence at comparable exposure</td>
<td>Reduced proliferation</td>
<td>Cell-cycle phase varies among studies and may reflect secondary effects of metabolic stress or apoptosis.</td>
</tr>
<tr>
<td>8</td>
<td>Migration invasion and EMT</td>
<td>↓ migration; ↓ invasion; ↓ mesenchymal phenotype (model-dependent)</td>
<td>Insufficient evidence</td>
<td>Reduced metastatic behaviour</td>
<td>Predominantly in-vitro evidence; clinically relevant anti-metastatic activity has not been demonstrated.</td>
</tr>
<tr>
<td>9</td>
<td>Mitochondrial ROS increase</td>
<td>↑ ROS and oxidative stress in some cytotoxic models (dose-dependent) (high concentration only)</td>
<td>↓ oxidative injury in multiple inflammatory or toxic-injury models (context-dependent)</td>
<td>Oxidative mitochondrial damage and apoptosis</td>
<td>Geraniol is not uniformly pro-oxidant. Redox direction depends on tissue, baseline stress, concentration, and treatment duration.</td>
</tr>
<tr>
<td>10</td>
<td>NRF2 antioxidant response</td>
<td>↔ or uncertain; possible cytoprotection in some contexts</td>
<td>↑ NRF2-associated antioxidant enzymes in selected injury models</td>
<td>Secondary antioxidant and tissue-protective response</td>
<td>NRF2 is not a well-established central anticancer mechanism for geraniol. Persistent NRF2 activation could theoretically protect some tumour cells.</td>
</tr>
<tr>
<td>11</td>
<td>5-Fluorouracil chemosensitization</td>
<td>↑ response to 5-fluorouracil; ↓ tumour growth in colorectal xenograft models</td>
<td>Insufficient selectivity data</td>
<td>Enhanced chemotherapy activity</td>
<td>Promising preclinical interaction, but human efficacy, optimal scheduling, toxicity, and pharmacokinetic interactions are unknown.</td>
</tr>
<tr>
<td>12</td>
<td>Clinical Translation Constraint</td>
<td>Rapid metabolism; transient free-geraniol exposure; many studies use high concentrations</td>
<td>Fragrance sensitization and irritation; systemic high-dose safety incompletely characterized</td>
<td>Limits direct translation of experimental cytotoxicity</td>
<td>Formulation strongly affects bioavailability. Citronella oil is a heterogeneous mixture and cannot be dosed or interpreted as purified geraniol.</td>
</tr>
</tbody>
</table>