Calendula: Traditional Use, Constituents and Evidence

My nan kept a row of calendula along the south-facing fence of her garden in Stirling, not far from the bottom of the Adelaide Hills. She called them "pot marigolds" , the old English name , and treated them the same way she treated comfrey and chamomile: things worth having, things with history. She never claimed they cured anything. She just knew they'd been used for a long time. That instinct, I've since learned, lines up surprisingly well with how modern phytomedicine researchers frame this particular plant.

Calendula officinalis L. (Asteraceae) has one of the longest continuous records of documented human use of any flowering annual. That doesn't make it medicine in any clinical sense. But it makes it worth understanding carefully.

A brief history of calendula in traditional medicine

The name comes from the Latin calendae , the first day of the month; because the plant flowers with almost mechanical regularity through much of the year in temperate climates. It was described in European herbals by at least the 12th century; Hildegard of Bingen noted it in her Physica. By the 16th century it appeared in English, German and Italian domestic medicine texts. John Gerard's Herball (1597) recommended a conserve of the flowers for "the trembling of the harte", which tells you more about early-modern humoral theory than it does about the plant, honestly, but the presence is consistent across centuries of documentation.

In Ayurvedic practice, a closely related species has been used in external preparations for skin. Traditional Arabic medicine incorporated calendula flowers in topical applications. Across the folk medicine record in Central Europe, the dominant pattern is external use: preparations applied to wounds, burns, and skin irritations. Internal preparations, teas, tinctures; appear in some European traditions too, especially for digestive complaints, though the evidence base here is much thinner.

This historical pattern matters because, as phytomedicine researchers often point out, traditional use concentrated in external application tends to correspond with constituents that are biologically active at the tissue surface. Calendula is a reasonable example of that principle.

Key constituents and their chemistry

Calendula flowers are chemically complex. The main constituent classes that appear in the research literature are:

• Triterpenoid saponins, oleanolic acid glycosides, including calendulosides. These are considered the marker compounds for most standardised extracts of calendula. They're water-soluble and relatively unstable, which affects how preparations are formulated and stored.
• Flavonoids, quercetin, isorhamnetin, and narcissin glycosides are among the most studied. Flavonoids are phenolic compounds found widely across the plant kingdom; their presence in calendula is unremarkable in isolation but contributes to the overall polyphenol load of any extract.
• Carotenoids; calendula's orange pigment comes largely from lutein, zeaxanthin, and beta-carotene. These are fat-soluble, which is why oil-based preparations are common in topical use.
• Polysaccharides, water-soluble fractions with high molecular weight. In vitro studies have characterised these extensively; their significance in complex extracts is harder to pin down.
• Essential oil constituents, the volatile fraction includes alpha-pinene, linalool, and small amounts of bisabolol, the same terpene found notably in German chamomile. The overall terpene content in calendula is relatively low compared to species like lavender or chamomile, so the essential oil is rarely the primary focus of research.

The interaction between these fractions; and whether a whole-flower preparation behaves differently from an isolated fraction, is an open question. Anyone familiar with the debate around the entourage effect in cannabis research will recognise the shape of that argument, even if the plant and the regulatory landscape are completely different.

What the research actually says, and what it doesn't

The short version is: there is a body of in vitro and animal research on calendula extracts, a smaller number of clinical trials, and a significant gap between the two.

In vitro studies; cells in a dish, have characterised the antioxidant activity of flavonoid and carotenoid fractions and explored how triterpenoid saponins interact with various cellular pathways. These studies are useful for understanding mechanism at a molecular level. They are not evidence of clinical effect in humans. The leap from a petri dish to a person is enormous, and the phytomedicine field has a habit of glossing over it in popular writing. I'd argue this is one of the more underappreciated problems in how botanical research gets communicated to the public.

Clinical trial data on calendula is modest in volume and mixed in quality. A 2013 randomised controlled trial published in the Journal of Clinical Oncology (Pommier et al.) compared a calendula-based preparation to trolamine for acute dermatitis in breast cancer patients receiving radiotherapy. The calendula arm showed statistically significant differences on several outcomes. That's one reasonably well-designed trial. It's also more than 10 years old and has not been replicated at scale. The bioavailability questions for topical preparations, how much of any given constituent actually reaches relevant tissue layers; remain incompletely understood.

For oral preparations, the evidence base is genuinely thin. Mechanistic hypotheses abound. High-quality clinical data in humans is scarce. Honest researchers say so.

Calendula and Australian regulation

In Australia, Calendula officinalis flower preparations are permitted ingredients in listed complementary medicines. The Therapeutic Goods Administration (TGA) maintains a list of permitted ingredients under the Australian Register of Therapeutic Goods (ARTG), and listed medicines (AUST L) go through a quality and safety assessment rather than a full efficacy review. This is an important distinction: a product bearing an AUST L number has been assessed for manufacturing quality and ingredient safety, it has not been assessed by the TGA as effective for any specific indication.

Products can include traditional-use claims, but these must be within the permitted indications framework and must not constitute therapeutic claims in the prohibited-representation sense. The TGA's advertising rules, applied through the Therapeutic Goods Advertising Code, are specific about this. Manufacturers walking the line between permitted traditional-use language and prohibited therapeutic claims are doing so in a regulated space with genuine consequences for getting it wrong.

Calendula appears in external preparations (creams, gels, ointments, oils) and internal preparations (teas, liquid extracts, tablets). Neither category requires a prescription; neither falls under the Poisons Standard in the way that Schedule 4 or Schedule 8 substances do. It's not a restricted import in the way that, say, fresh kava plant material is (kava powder for personal use has specific import controls under Australian Border Force regulations that make the calendula situation look uncomplicated by comparison).

Good Manufacturing Practice (GMP) requirements apply to all Australian-manufactured and imported listed medicines. This matters more than it sounds: the quality and standardisation of botanical extracts varies enormously between manufacturers, and the constituent profile of a product not made to GMP is genuinely unpredictable.

Standardisation and what it means in practice

The phrase "standardised to X% triterpenoids" on a calendula extract label means the manufacturer has set a minimum or fixed level of the marker compound, usually oleanolic acid derivatives, in the finished product. Standardised extracts allow for reproducibility between batches, which matters both for research and for consumer consistency.

But standardisation to one marker compound doesn't tell you the full picture of what's in the extract. A preparation could be standardised to triterpenoids while varying considerably in its flavonoid and carotenoid content depending on growing conditions, harvest time, and processing method. Growing calendula in the Adelaide Hills in autumn gives a different pigment load than growing it in July in a glasshouse. I know this from watching the flower colour shift in my nan's garden across the seasons, and the chemistry literature confirms the variation is real and significant.

Researchers working with calendula in trials now generally specify the extract ratio, the solvent used, the marker compound and its concentration, and ideally the full constituent profile. Studies that don't do this are harder to interpret and harder to replicate.

The honest position

Calendula has a genuine, documented history in traditional plant medicine across multiple cultures. Its constituent chemistry is reasonably well characterised. Early-stage research has explored mechanisms at the cellular level. Clinical trial data in humans is limited in volume and scope, and a cautious reading of the evidence base should acknowledge that gap plainly.

For anyone interested in how botanical research methodology works more broadly; the relationship between in vitro data, animal studies, and human clinical evidence, calendula is a useful case study. It sits right at the intersection of rich traditional use and incomplete modern evidence, which is exactly where most of phytomedicine lives.

Nan would have found the whole debate a bit exhausting, I think. She just kept planting the things.

Sources

• Listed Medicines Evidence Guidelines, Therapeutic Goods Administration (TGA)
• Pommier et al. (2004), Phase III Randomized Trial of Calendula Officinalis vs Trolamine; NCBI/PubMed
• Calendula officinalis: Molecular Basis of Biological Activity, NCBI/PMC
• Permissible Indications for Listed Medicines, Therapeutic Goods Administration (TGA)

, Naomi Ellison, Botanical & phytomedicine writer