Marigold is Calendula officinalis flower. The name is derived from the Latin word calendae, meaning the first day of the month.

Carotenoids and other compounds in Calendula herb
Calendula flower has several carotenoids including lutein and zeaxanthin. Orange and yellow calendula petals have lycopene, gamma-carotene and rubixanthin. Other substances include triterpene glycosides, flavonol glycosides, oleanolic acid, beta-amyrin, beta-amyrin acetate, rutin, narcissin, 3-glucoside of isorhamnetin, quercetin, isoquercitrin, vanillic acid, caffeic acid, chlorogenic acid, protokatechuic acid, p-coumaric acid and syringic acid.

Anti-inflammatory benefits
Preparative purification of the major anti-inflammatory triterpenoid esters from Marigold (Calendula officinalis).
Fitoterapia. 2003.
A method for the efficient preparative purification of faradiol 3-O-laurate, palmitate and myristate, the major anti-inflammatory triterpenoid esters in the flower heads of the medicinal plant Calendula officinalis has been developed. Gram quantities of the individual compounds were obtained with 96 to 98% purity by a combination of supercritical fluid extraction (SFE), normal-phase and reversed-phase column chromatography. During the work-up of the faradiol esters, accompanying minor compounds of the triterpene ester fraction were purified and identified by spectroscopic means as maniladiol 3-O-laurate and myristate.

Antioxidant
Protective properties of butanolic extract of the Calendula officinalis against lipid peroxidation of rat liver microsomes and action as free radical scavenger.
Redox Report. 2002.
Calendula officinalis (marigold) has many pharmacological properties. It is used for the treatment of skin disorders, pain and also as a bactericide, antiseptic and anti-inflammatory. Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are known to participate in the pathogenesis of various human diseases and may be involved in the conditions which C. officinalis is used to treat. The aim of this study was to investigate the relationship between the beneficial properties of this plant and its antioxidant action. The butanolic fraction (BF) was studied because it is non-cytotoxic and is rich in a variety of bioactive metabolites including flavonoids and terpenoids. The results obtained suggest that the butanolic fraction of C. officinalis possesses a significant free radical scavenging and antioxidant activity and that the proposed therapeutic efficacy of this plant could be due, in part, to these properties.

Calendula officinalis health benefit
At this time I have not seen any studies to indicate the health benefit of ingesting a calendula herb capsule. Eclectic Institute offers a Calendula supplement containing 250 mg of the flower and a calendula liquid tincture. My guess is that an oral calendula supplement would offer antioxidant benefits, but I don't know what kind of medicinal properties it would have.

Calendula for abdominal spasms
Studies on spasmogenic and spasmolytic activities of Calendula officinalis flowers.
Phytotherapy Res. 2006.
The aqueous-ethanol extract of Calendula officinalis flowers was studied for its possible spasmolytic and spasmogenic effects in isolated gut preparations. In rabbit jejunum, Calendula officinalis flowers caused a dose-dependent relaxation of spontaneous and K+-induced contractions, suggestive of calcium channel blockade. In a few preparations, a mild non-reproducible spasmogenic effect was observed at lower doses, followed by relaxation. The calcium channel blockade effect was confirmed when pretreatment of the jejunum preparations with Calendula officinalis flowers produced a dose-dependent rightward shift in the Ca(++) dose-response curves, similar to that of verapamil. Activity-directed fractionation revealed that the spasmolytic activity of the plant was concentrated in its organic fractions. The aqueous fraction exhibited a marked atropine sensitive spasmogenic effect but was found to be devoid of any spasmolytic effect. These data indicate that the crude extract of Calendula officinalis flowers contains both spasmolytic and spasmogenic constituents, exhibiting these effects through calcium channel blocking and cholinergic activities and this study provides a scientific base for its traditional use in abdominal cramps and constipation.

Calendula anti-tumor activity
A new extract of the plant Calendula officinalis produces a dual in vitro effect: cytotoxic anti-tumor activity and lymphocyte activation.
BMC Cancer. 2006.
Phytopharmacological studies of different Calendula extracts have shown anti-inflammatory, anti-viral and anti-genotoxic properties of therapeutic interest. In this study, we evaluated the in vitro cytotoxic anti-tumor and immunomodulatory activities and in vivo anti-tumor effect of Laser Activated Calendula Extract (LACE), a novel extract of the plant Calendula Officinalis (Asteraceae). An aqueous extract of Calendula Officinalis was obtained by a novel extraction method in order to measure its anti-tumor and immunomodulatory activities in vitro. Tumor cell lines derived from leukemias, melanomas, fibrosarcomas and cancers of breast, prostate, cervix, lung, pancreas and colorectal were used and tumor cell proliferation in vitro was measured by BrdU incorporation and viable cell count.  These results indicate that LACE aqueous extract has two complementary activities in vitro with potential anti-tumor therapeutic effect: cytotoxic tumor cell activity and lymphocyte activation. Calendula extract presented in vivo anti-tumoral activity in nude mice against tumor growth of Ando-2 melanoma cells.

Calendula Research
J Indian Soc Periodontol. 2013. Evaluation of Calendula officinalis as an anti-plaque and anti-gingivitis agent. Within the limits of this study, it can be concluded that calendula mouthwash is effective in reducing dental plaque and gingivitis adjunctive to scaling.

Phase III randomized trial of Calendula officinalis compared with trolamine for the prevention of acute dermatitis during irradiation for breast cancer.
J Clin Oncol. 2004.
The effectiveness of nonsteroid topical agents for the prevention of acute dermatitis during adjuvant radiotherapy for breast carcinoma has not been demonstrated. The goal of this study was to compare the effectiveness of calendula (Pommade au Calendula par Digestion; Boiron Ltd, Levallois-Perret, France) with that of trolamine (Biafine; Genmedix Ltd, France), which is considered in many institutions to be the reference topical agent. Between July 1999 and June 2001, 254 patients who had been operated on for breast cancer and who were to receive postoperative radiation therapy were randomly allocated to application of either trolamine (128 patients) or calendula (126 patients) on the irradiated fields after each session. The primary end point was the occurrence of acute dermatitis of grade 2 or higher. Prognostic factors, including treatment modalities and patient characteristics, were also investigated. Secondary end points were the occurrence of pain, the quantity of topical agent used, and patient satisfaction. The occurrence of acute dermatitis of grade 2 or higher was significantly lower (41% v 63%) with the use of calendula than with trolamine. Moreover, patients receiving calendula had less frequent interruption of radiotherapy and significantly reduced radiation-induced pain. Calendula was considered to be more difficult to apply, but self-assessed satisfaction was greater. Body mass index and adjuvant chemotherapy before radiotherapy after lumpectomy were significant prognostic factors for acute dermatitis. Calendula is highly effective for the prevention of acute dermatitis of grade 2 or higher and should be proposed for patients undergoing postoperative irradiation for breast cancer.

HPLC study on the carotenoid composition of Calendula products.
J Biochem Biophys Methods. 2002.
The authors report on the HPLC investigation of the carotenoid composition of the steams, leaves, petals and pollens of Calendula officinalis L. In the petals and pollens, the main carotenoids were flavoxanthin and auroxanthin while the stem and leaves mostly contained lutein and beta-carotene. Five different herbal tea and two tinctures made from the flower of C. officinalis were also investigated and the carotenoid composition of the industrial products was compared to the starting material.

Side effects, safety
Final report on the safety assessment of Calendula officinalis extract.
Int J Toxicol. 20010.
Calendula Officinalis extract is an extract of the flowers of the common marigold. Techniques for preparing Calendula Officinalis Extract include gentle disintegration in soybean oil. Propylene glycol and butylene glycol extractions were also reported. Components of these ingredients are variously reported to include sugars, carotenoids, phenolic acids, sterols, saponins, flavonoids, resins, sterins, quinones, mucilages, vitamins, polyprenylquinones, and essential oils. Calendula Officinalis Extract is reported to be used in almost 200 cosmetic formulations, over a wide range of product categories. There are no reported uses of Calendula Officinalis. Acute toxicity studies in rats and mice indicate that the extract is relatively nontoxic. Animal tests showed at most minimal skin irritation, and no sensitization or phototoxicity. Minimal ocular irritation was seen with one formulation and no irritation with others. Six saponins isolated from C. officinalis flowers were not mutagenic in an Ames test, and a tea derived from C. officinalis was not genotoxic in Drosophila melanogaster. No carcinogenicity or reproductive and developmental toxicity data were available. Clinical testing of cosmetic formulations containing the extract elicited little irritation or sensitization. Absent any basis for concluding that data on one member of a botanical ingredient group can be extrapolated to another in a group, or to the same ingredient extracted differently, these data were not considered sufficient to assess the safety of these ingredients. Additional data needs include current concentration of use data; function in cosmetics; ultraviolet (UV) absorption data; if absorption occurs in the UVA or UVB range, photosensitization data are needed; gross pathology and histopathology in skin and other major organ systems associated with repeated dermal exposures; dermal reproductive/developmental toxicity data; inhalation toxicity data, especially addressing the concentration, amount delivered, and particle size; and genotoxicity testing in a mammalian system; if positive, a 2-year dermal carcinogenicity assay performed using National Toxicology Program (NTP) methods is needed. Until these data are available, it is concluded that the available data are insufficient to support the safety of these ingredients in cosmetic formulations.

Anti-HIV activity of extracts from Calendula officinalis flowers.
Biomed Pharmacother. 1997.
Extracts of dried flowers were examined for their ability to inhibit the human immunodeficiency virus type 1 (HIV-1) replication. Both organic and aqueous extracts were relatively nontoxic to human lymphocytic Molt-4 cells, but only the organic one exhibited potent anti-HIV activity.