Active Compounds in Herbs: Alkaloids, Flavonoids, Terpenes, Glycosides and Tannins
Plain definitions of alkaloids, flavonoids, terpenes, glycosides and tannins in plants, with the point that a compound class is not a health claim and some compounds are drugs or poisons.
Content last updated . This page has not been medically reviewed.
Safety first. Natural products can cause side effects and interact with medicines. Read herb safety and interactions before you use anything on this page.
What "active compound" means
Plants make thousands of chemicals that are not needed for basic growth. A scientific chapter on plant secondary metabolites groups them into families such as alkaloids, terpenes, flavonoids, phenols (which include tannins) and glycosides.[1] Chemists name these families by structure, not by what they do to a person.
This page defines the five families in plain words. It makes no health claim for any family. Knowing that a plant contains a flavonoid or an alkaloid does not tell you that a product is useful or safe, and the same family can hold both medicines and poisons. It is information only.
The five families
| Family | Plain definition from the source | Why it matters |
|---|---|---|
| Alkaloids | Plant-derived compounds containing one or more nitrogen atoms.[1] | Many alkaloids act on the nervous system, and improperly applied, some stimulants and sedatives are deadly.[1] |
| Terpenes | Compounds that all possess repeating five-carbon isoprene units.[1] | The antimalarial medicine artemisinin is a terpene-type compound from a Chinese medicinal plant.[1] |
| Flavonoids | A large family of compounds, estimated as exceeding 10,000.[1] | Size of the family says nothing about benefit; each compound and product needs its own evidence. |
| Glycosides | The source describes cardiac glycosides as consisting of a sugar molecule bound to a steroid.[1] | Cardiac glycosides, such as the one from foxglove named in the source, are used to treat cardiac insufficiency, so they are drugs, not foods.[1] |
| Tannins | Phenols range from simple compounds with a single aromatic ring to complex polymers like tannins and lignins.[1] | A grouping by structure only; it does not predict an effect. |
Compound classes are not health claims
It is common for web pages to say "rich in flavonoids" as if that were a benefit. We do not make that claim. The evidence for a plant comes from studies of that plant part and preparation in people, not from the compound class. Where we looked, a class name alone did not support a use: Proper evidence not found for a class-level health claim in the sources we read. We searched the chapter above and the NCCIH fact sheets used on this site.
Two cautions from the source material:
- The same chapter notes that single species of plants are not homogenous in their chemical content.[1] Two samples of one plant can differ.
- NCCIH reports research suggesting the activity of echinacea extracts is influenced by soil conditions that affect the plant's bacterial community.[2] Growing conditions can change what is in a batch.
Why standardized extracts name a marker
A product may be "standardized" to a marker compound so that batches can be compared. That tells you a number about one compound, not that the product works. See standardized herbal extracts and standardization and marker compounds.
Related pages
Frequently asked questions
Are alkaloids always poisonous?
No. The source says many act on the nervous system and some stimulants and sedatives are deadly if improperly applied, yet it also names medicines among them, such as codeine used by doctors to suppress severe coughing.[1] Each compound is judged on its own evidence.
What is a flavonoid?
A member of a very large family of plant compounds, estimated as exceeding 10,000.[1] We do not claim a benefit for the family.
Is a tannin the same as a polyphenol?
The source describes tannins as polymers within the phenol group.[1] We did not read a source that sets out the wider naming rules, so we do not go further.
Does "contains a glycoside" mean a herb is a drug?
Some glycosides are drugs: the source names cardiac glycosides from foxglove used to treat cardiac insufficiency.[1] Others are not. Check the specific herb page.
Can I judge a herb by its compound list?
No. Samples of the same species can differ,[1] and growing conditions can change extract activity.[2] Ask a pharmacist, and see reading supplement labels.
More details: evidence, limits and cautions
The details behind this page: cautions, limits of the evidence and where sources disagree. Open any note to read it.
When a compound is the risk
Documented harms often come from one compound class, and regulators name them.
- Glycosides in licorice: NCCIH says licorice contains glycyrrhizin, which can cause serious adverse effects, such as irregular heartbeat and cardiac arrest.[3] See licorice root for the full caution.
- Pyrrolizidine alkaloids in comfrey: in a July 2001 letter to industry groups, the US Food and Drug Administration (FDA) said comfrey is a source of pyrrolizidine alkaloids that present a serious health hazard when ingested.[4]
- Aristolochic acids: the US National Cancer Institute (NCI) advises people to not use herbal products that contain aristolochic acids.[5]
- Ephedrine alkaloids: the FDA banned dietary supplements containing them in the United States in 2004.[6]
Safety and who should ask a clinician
A single compound class in a food is different from a concentrated extract. Anyone on prescription medicines, pregnant or breastfeeding, with liver or kidney disease, or having surgery should ask a pharmacist or clinician before using a concentrated herbal product. See medicine interactions and herbs and liver injury.
References
- Secondary Metabolites of Plants (book chapter, Springer; PMC7123774). Secondary Metabolites of Plants (book chapter, Springer; PMC7123774). Read 2026-10-23.
- NCCIH, Echinacea (US). NCCIH, Echinacea (US). Read 2026-10-23.
- NCCIH, Licorice Root (US). NCCIH, Licorice Root (US). Read 2026-10-23.
- FDA, letter advising supplement manufacturers to remove comfrey products from the market, 6 July 2001 (US; archived copy). FDA, letter advising supplement manufacturers to remove comfrey products from the market, 6 July 2001 (US; archived copy). Read 2026-10-23.
- National Cancer Institute, Aristolochic Acids (US). National Cancer Institute, Aristolochic Acids (US). Read 2026-10-23.
- NCCIH, Ephedra (US). NCCIH, Ephedra (US). Read 2026-10-23.