Benefits
Occasional heartburn: patent-reported studies
The heartburn figures come from two small studies in a US patent written by its inventor, later restated in a review but never published as a peer-reviewed trial. In the first, 19 adults took 1,000 mg daily or every other day with no control group. In the second, 86% of the d-limonene group (6 of 7) and 29% of the placebo group reported relief by day 14. The proposed acid and motility mechanisms have not been tested in people.
High-dose research in people with cancer (not a supplement use)
Cancer research has given oral d-limonene at doses above typical supplement servings, and it is not a treatment for any disease. A dose-finding trial in 32 people with advanced tumours set the maximum tolerated dose at 8 g/m2/day; its 10-person breast cancer extension had no responses. In an open-label study without a placebo group, 43 women took 2 g daily before breast surgery and limonene concentrated in breast tissue.
Blood inflammation markers in older adults
In an open-label trial, 125 healthy adults aged 65 to 85 followed a Mediterranean-style diet for 56 days, alone or with one of three supplements. The primary outcome, hsCRP, did not change in any arm. In the 30 people also taking an orange-peel d-limonene softgel at about 10 mg/kg/day, fibrinogen fell more than with diet alone or with argan oil. No symptom outcome was measured.
Gallstone solvent: a hospital procedure, not an oral supplement use
This has nothing to do with swallowing a capsule. Clinical reports, mostly from Japan, describe doctors infusing a limonene solvent directly into the gallbladder or bile ducts to dissolve cholesterol-containing stones. Taking oral d-limonene has not been shown to dissolve gallstones.
Mechanism of action
Proposed mucosal coating and motility effects (laboratory only)
The patent and a 2007 review propose that limonene coats the stomach lining against acid, citing in vitro work that neither describes, and that it supports peristalsis, citing increased resting tone in guinea pig ileum and rat vas deferens tissue. Neither effect has been measured in people, and neither source reports any test of an effect on the lower oesophageal sphincter.
Protein farnesylation inhibition (a laboratory finding)
D-limonene inhibits protein isoprenylation, the attachment of lipid groups that many growth-regulating proteins need, and a 2007 review suggests this may explain its antitumour activity in cell culture and rodent tumour models. It has never been shown to prevent or treat cancer in people, and the exposures involved are far above what a supplement serving delivers.
Antioxidant and anti-inflammatory mechanisms (cell and animal work)
Cell and animal studies, collected in a 2018 review, report antioxidant and anti-inflammatory activity for limonene. In cultured fibroblasts it reduced the movement of NF-kB, a switch for inflammatory genes, into the cell nucleus. These laboratory effects have not been confirmed in people taking oral d-limonene.
TRPA1 channel activity (animal and cell work only)
In mouse sensory neurons and cells expressing the channel, limonene activated TRPA1 but not TRPV1. Injected into the paw it caused brief pain, while given systemically to mice it reduced pain behaviour triggered by oxidative stress. A study of the human channels in cultured cells found no effect of limonene on TRPA1 or TRPV1. None of this has been tested in the human gut.
Clinical trials
Phase I dose-escalation with a limited phase II extension (Vigushin et al. 1998, Cancer Chemother Pharmacol)
32 patients with refractory solid tumors completing 99 courses of D-limonene 0.5-12 g/m²/day orally in 21-day cycles. Pharmacokinetics analyzed by LC-MS. Additional 10 breast cancer patients received 15 cycles at 8 g/m²/day. Intratumoral monoterpene levels measured in 2 patients.
The maximum tolerated dose was 8 g/m2/day; nausea, vomiting and diarrhoea limited the dose. One breast cancer patient on 8 g/m2/day had a partial response lasting 11 months, and three colorectal cancer patients had prolonged stable disease. There were no responses in the phase II extension. Perillic acid, dihydroperillic acid and limonene-1,2-diol were the main circulating metabolites, and limonene levels in tumour tissue exceeded plasma levels in the two patients measured. These are chemotherapy dose-finding doses, far above any supplement serving.
Narrative review (Sun 2007, Altern Med Rev)
No participants: a review of d-limonene safety data and reported uses (gallstone dissolution, heartburn, cancer research), written by an author at Thorne Research, a supplement manufacturer.
Reports low toxicity in humans after single and repeated dosing for up to one year, and explains that the kidney tumours seen in male rats depend on alpha-2u-globulin, a protein people do not have. Its gallstone section describes solvent infused into the gallbladder or bile ducts, not swallowed. Its heartburn section restates the patent studies and misreports one figure: it says 89% achieved complete relief by day 14, while the patent reports 89% with ratings of 1 to 2 and 79% with complete relief.
Inventor-reported studies, Examples 1 and 2 (Wilkins 2002, US patent 6,420,435 B1). Restated in a 2007 review but not published as a peer-reviewed trial.
Study 1: 19 adults with heartburn or reflux symptoms for at least five years, with no control group, taking 1,000 mg d-limonene every day or every other day for up to 20 days. The text assigns 11 to every-other-day and 5 to daily dosing, only 16 people; the table percentages fit groups of 12 and 7. Study 2: double blind, 13 adults, described as 7 on d-limonene and 6 on a soybean-oil placebo, though the table states n=14 and its placebo percentages fit a group of seven.
Study 1: 32% reached severity ratings of 1 to 2 (none or mild) by day 2 and 89% by day 14; 79% reported complete relief by day 14. Study 2: in the d-limonene group 29% improved by days 4 and 7, 71% by day 9 and 86% (6 of 7) by day 14, against 0%, 14%, 29% and 29% on placebo. The patent calls the difference significant but reports no statistical test, Study 1 had no control group, and the inventor wrote the document in support of a patent claim.
Double-blind, randomized, placebo-controlled trial, NCT04484467 (Ivashkin et al. 2022, PLoS One)
56 adults with irritable bowel syndrome, some also with functional dyspepsia, all continuing standard therapy and given the combination supplement or a placebo for 30 days.
Total 7x7 symptom scores did not differ between groups at the middle visit (p = 0.4) and favoured the supplement group at the final visit (p = 0.009). Gut microbiota showed no statistically significant changes, and no obvious side effects were reported. Limonene was one of three active ingredients, so this is combination evidence and does not show what limonene alone does.