Calendula may cause contact dermatitis, or an anaphylactic or eczematous allergic reaction.
– Contact Dermatitis 2001 Nov;45(5):269-72 — The seamy side of natural medicines: contact sensitization to arnica (Arnica montana L.) and marigold (Calendula officinalis L.). — Reider N, Komericki P, Hausen BM, Fritsch P, Aberer W.

Concluded that the available data are insufficient to support the safety of these ingredients in cosmetic formulations.
– Int J Toxicol 2001;20 Suppl 2:13-20 — Final report on the safety assessment of Calendula officinalis extract and Calendula officinalis.

Calendula may affect the menstrual cycle and should not be used during pregnancy or nursing.

Calendula may have a sedative action that can interact with medications.
– Farmakol Toksikol 1978 Sep-Oct;41(5):556-60 — Pharmacology of calenduloside B, a new triterpene glycoside from the roots of Calendula officinalis — Iatsyno AI, Belova LF, Lipkina GS, Sokolov SI, Trutneva EA.

People with allergies to plants in the Aster/Compositae family, such as ragweed, chrysanthemums, marigolds, and daisies, are more likely to have an allergic reaction to calendula. In early animal studies, high doses of calendula preparations were reported to lower blood pressure. It is not clear if the use of calendula on the skin of humans has this effect. In theory, the use of calendula in combination with drugs that lower blood pressure may lead to increased effects. Calendula may also increase the effects of antispasmodics, which are drugs that help stop muscle spasms. Other possible interactions include increases in the activity of hypoglycemic medications or insulin, antifungal medications, or drugs that lower cholesterol. It is not clear if calendula is safe for use during pregnancy or breastfeeding.
– Source: NaturalStandard.com