The short answer

Limonene is a monoterpene with the formula C10H16. It gives citrus peel its smell. In cannabis flower it often lands among the top five terpenes by volume. Lab tests and animal models link it to mood effects, stress reduction, and antibacterial action. Human data is thin. Most human trials used oral capsules of 1 g to 2 g per day, not inhaled flower.

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What limonene is

Limonene is a cyclic monoterpene. The molecule has two mirror forms. Cannabis and citrus carry d-limonene (CAS 5989-27-5), the form with the lemon odor. L-limonene smells of pine and turpentine and is rare in cannabis.

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Boiling point: 176 C (349 F). It evaporates at room temperature. That is why open jars of flower lose terpene content and why citrus peel smells strong.

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Where it appears

Cultivars sold as lemon, citrus, or orange often list limonene as the top terpene on a lab report.

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Reported effects

Mood and stress

Inhalation of citrus oil lowered anxiety scores in small aromatherapy trials. Mouse studies reported antidepressant-like and anxiolytic-like effects. One proposed route is adenosine A2A receptor signaling, not the CB1 receptor. Limonene does not produce a high.

Digestion

A 2007 review described a pilot study of 32 patients with gastroesophageal reflux. Symptoms cleared within 2 weeks in most of them after 1 g of d-limonene per day. No large trial has repeated that result.

Antimicrobial action

Limonene inhibits several bacteria and fungi in petri dish tests, including Staphylococcus aureus and Candida albicans. Test concentrations sit far above what a person reaches by inhaling vapor.

Cell studies

Limonene and its metabolites, perillic acid and dihydroperillic acid, slow cancer cell growth in culture. A phase I trial in advanced cancer used 2 g per square meter of body surface per day. The trial recorded stable disease in a few patients and no tumor shrinkage. Oral limonene is not a cancer treatment.

How the body handles it

The liver enzymes CYP2C9 and CYP2C19 convert limonene to perillic acid. Blood levels peak within 1 to 2 hours after an oral dose and drop over the next several hours. Inhaled limonene reaches the bloodstream at lower levels than a capsule.

Safety

The FDA lists d-limonene as GRAS for food use. Trials used up to 2 g per day for weeks. Reported side effects were mild nausea and diarrhea. Oxidized limonene, not fresh limonene, causes skin irritation. Old citrus oil that smells sharp and turpentine-like has oxidized. Keep it off skin.

Limonene is a solvent. It strips paint and dissolves polystyrene. That is why it shows up in cleaning products.

Use with cannabis

No standard dose exists for inhaled limonene. Vaporization starts near 176 C. Higher temperatures degrade terpenes and can add irritants to the vapor. Some users report that limonene-heavy flower smooths THC anxiety. Controlled human tests of that claim do not exist.

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