Herbal medicine is the use of plants and plant-derived preparations to prevent or treat health conditions. It is the oldest and most widespread form of medicine in the world, and it remains deeply embedded in modern healthcare: roughly 50% of pharmaceutical drugs approved over the last 30 years trace their origins directly or indirectly to natural plant compounds. The World Health Organization defines herbal medicines as products “that contain as active ingredients parts of plants, other plant materials, or combinations thereof,” including raw herbs, processed herbal materials, and finished products like capsules or teas.
How Plants Produce Medicinal Compounds
Plants don’t make biologically active chemicals for human benefit. They produce them as defense mechanisms against insects, fungi, and grazing animals, or to attract pollinators. These compounds, called secondary metabolites, happen to interact with human biology in powerful ways because insects and mammals share many of the same basic cellular machinery. There are three major families of these compounds, and each works differently in the body.
Alkaloids are a group of over 12,000 nitrogen-containing compounds. Caffeine and nicotine are both alkaloids. They work by binding directly to receptor sites on nerve cells, interfering with the enzymes that break down chemical messengers in the brain, or altering how signals pass through cell membranes. This is why alkaloid-containing plants tend to have strong, noticeable effects on alertness, mood, or pain.
Terpenes number more than 30,000 known compounds and are fat-soluble, meaning they cross into tissues easily. Plants like ginkgo, ginseng, valerian, and sage are rich in terpenes. Many of these compounds interact with the body’s calming signaling systems and with the enzymes that regulate attention and memory.
Phenolic compounds, especially the subgroup called flavonoids, are the largest and most diverse class. They show up in turmeric (as curcumin), green tea, grapes (as resveratrol), and soy. Different flavonoids can produce calming effects by binding to the same receptors targeted by anti-anxiety medications, enhance cognition by boosting certain brain chemicals, or act as antidepressants by slowing the breakdown of mood-regulating molecules like serotonin and dopamine. Some phenolic compounds also mimic estrogen in the body, which is why soy and similar plants have effects on hormonal health.
Herbs With Strong Clinical Evidence
Not every herbal remedy has rigorous science behind it, but several have been evaluated in clinical trials large enough to draw meaningful conclusions. A review in the Journal of General Internal Medicine rated the following among the most commonly used herbs in the United States and assessed their evidence:
- St. John’s wort is rated “likely effective” for mild to moderate depression. It works by influencing serotonin and other mood-related brain chemicals in ways similar to standard antidepressants.
- Ginkgo biloba is rated “likely effective” for dementia symptoms and for claudication, a condition involving poor blood flow to the legs that causes pain during walking.
- Garlic is rated “likely effective” for high cholesterol. Regular supplementation appears to modestly reduce total cholesterol levels.
- Kava kava is rated “likely effective” for anxiety. Its active compounds interact with calming receptor systems in the brain.
- Soy is rated “effective” for high cholesterol but “not effective” for menopausal symptoms like hot flashes, despite widespread marketing for that purpose.
These ratings reflect the weight of available trial data, not certainty. “Likely effective” means multiple well-designed studies support the use, but the evidence isn’t as ironclad as what’s required for prescription drug approval.
Common Forms and Preparations
The way an herb is prepared matters because different methods extract different compounds. A simple tea (or infusion) uses hot water and pulls out water-soluble ingredients effectively, but it will miss resinous or oily compounds that don’t dissolve in water. Decoctions, which involve simmering tougher plant parts like roots and bark, extract a broader range of water-soluble chemicals through prolonged heat.
Tinctures use alcohol as the solvent, which can capture compounds that water alone cannot reach, including resins and alkaloids. The concentration of alcohol varies depending on the plant. Herbs with more water-soluble components need a lower-proof alcohol (around 40 to 50%), while those with fewer water-soluble ingredients require high-proof alcohol (up to 90%) to pull out their active chemicals effectively. Tinctures are concentrated, so they’re taken in small doses, typically measured in drops.
Standardized extracts are manufactured to contain a guaranteed percentage of a specific active compound. A ginkgo extract standardized to 24% flavone glycosides, for example, delivers a consistent dose regardless of natural variation between plant batches. Capsules, tablets, powders, and topical preparations round out the options, each suited to different herbs and uses.
How Herbal Products Are Regulated
In the United States, herbal products are classified as dietary supplements, not drugs. This distinction has major practical consequences. Under the Dietary Supplement Health and Education Act (DSHEA), the FDA does not approve herbal supplements before they reach store shelves. Unlike prescription medications, which must be proven safe and effective before marketing, herbal products can be sold without submitting any safety or efficacy data to the FDA in advance. The exception is products containing a genuinely new dietary ingredient, which require the manufacturer to notify the FDA at least 75 days before selling the product.
Labels must identify the product as a dietary supplement, list each ingredient and its amount per serving, and include a domestic phone number or address for reporting serious side effects. When a company makes a claim about what the product does (such as “supports immune health”), the label must carry a disclaimer stating the product is “not intended to diagnose, treat, cure, or prevent any disease.” The FDA’s role is primarily reactive: it monitors the marketplace, investigates adverse event reports, and takes enforcement action after problems surface.
Because of this system, quality varies widely between brands. Third-party verification programs, like the one run by the U.S. Pharmacopeia (USP), test products for ingredient identity, purity, potency, and contamination. A USP-verified seal on a bottle means an independent lab has confirmed the product contains what the label says, in the amounts listed, without harmful contaminants. Looking for third-party seals (USP, NSF, or ConsumerLab) is one of the most practical steps you can take when choosing an herbal product.
Herb-Drug Interactions
Because herbal compounds are biologically active, they can interfere with prescription medications. Most of these interactions happen through the liver’s drug-processing enzymes, particularly a family of enzymes responsible for breaking down a large share of common medications. When an herb speeds up these enzymes, your body clears a drug faster than expected, making it less effective. When an herb slows them down, drug levels build up higher than intended, increasing the risk of side effects.
St. John’s wort is the most well-documented example. Its active compound has a strong affinity for a receptor in the liver that ramps up production of the enzyme responsible for processing a huge number of drugs, including birth control pills, blood thinners, HIV medications, and immunosuppressants. Taking St. John’s wort alongside these medications can reduce their effectiveness enough to cause real harm.
Goldenseal significantly inhibits two major drug-processing enzymes. In studies, goldenseal supplementation increased blood levels of a test drug by roughly 63%, a substantial jump that could make certain medications dangerously potent. Garlic oil, taken regularly over several weeks, reduces the activity of another liver enzyme by about 22 to 39%, depending on age. Even echinacea, widely considered mild, alters the activity of multiple liver enzymes in different directions simultaneously, inhibiting some while boosting others.
Ginkgo biloba appears to have relatively modest effects on these enzyme systems at normally recommended doses, though some interaction potential exists. The practical takeaway: if you take any prescription medication, herbal products are not automatically safe to add. The interactions are real, measurable, and in some cases clinically serious.
Herbal Medicine’s Role in Modern Healthcare
The relationship between herbal and conventional medicine is not the either-or divide it’s often presented as. Many standard pharmaceuticals were developed by isolating and refining compounds first discovered in plants. Aspirin originated from willow bark. The cancer drug paclitaxel comes from Pacific yew trees. Morphine is derived from opium poppies. Herbal medicine represents the broad, whole-plant end of a spectrum that extends to single-molecule pharmaceuticals at the other end.
What distinguishes herbal preparations from pharmaceuticals is complexity. A single herb may contain dozens of active compounds that work together, sometimes enhancing each other’s effects and sometimes buffering side effects. This complexity is both the appeal and the challenge: it makes standardization harder, clinical trials more difficult to design, and interactions harder to predict. But it also means herbal preparations can have a different therapeutic profile than an isolated compound, which is why the two approaches sometimes complement each other rather than compete.