The medicinal parts of St. John’s wort are the flowering tops, which include the flowers and upper leaves. These are the portions harvested, dried, and processed into the supplements, teas, tinctures, and oils you’ll find in stores. The stems and roots are not used medicinally.
Why the Flowering Tops Matter
The flowers and leaves of St. John’s wort (Hypericum perforatum) contain the plant’s two key active compounds: hypericin and hyperforin. Hypericin is the pigment responsible for the plant’s characteristic red-purple stain when you crush a flower bud between your fingers. Hyperforin is present in much higher concentrations, roughly 10 to 30 times more abundant than hypericin in dried plant material. Together, these compounds are what give the plant its antidepressant and anti-inflammatory properties.
If you look closely at the leaves, you’ll notice tiny translucent dots when you hold them up to light. These are oil glands containing the active compounds, and they’re the reason the plant’s Latin name includes “perforatum,” meaning perforated. The flowers also contain dark glands along the petal edges that are rich in hypericin.
How the Active Compounds Work
Hyperforin is considered the primary antidepressant component. It works by increasing the availability of several chemical messengers in the brain, including serotonin, norepinephrine, and dopamine. Rather than blocking the recycling of these neurotransmitters directly (the way many prescription antidepressants do), hyperforin appears to work indirectly by changing sodium levels inside nerve cells, which then affects how neurotransmitters are handled. The result is similar: more of these mood-regulating chemicals remain active in the brain for longer.
Hypericin contributes additional biological activity, though its exact role in mood effects is less clear-cut. It does, however, serve as a useful quality marker. Supplement labels typically list hypericin content as the standardization benchmark.
What “Standardized” Means on the Label
When you buy a St. John’s wort supplement, you’ll often see it labeled as standardized to a specific percentage of hypericin. The European Pharmacopoeia sets the benchmark: dried St. John’s wort herb should contain a minimum of 0.08% total hypericins, while concentrated dry extracts should fall between 0.10% and 0.30%. Most clinical trials have used extracts standardized to around 0.3% hypericin.
Hyperforin content also matters. The European standard caps it at 6.0% in standardized dry extracts. This is worth paying attention to because hyperforin is the compound most responsible for drug interactions (more on that below), so some manufacturers intentionally produce low-hyperforin extracts for people concerned about that risk.
When to Harvest for Maximum Potency
If you’re growing St. John’s wort yourself, timing the harvest is important. The plant is most potent when picked at full bloom, typically around midsummer, which traditionally falls near the feast of St. John the Baptist on June 24. This is where the common name comes from. Harvesting before or after peak flowering means lower concentrations of the active compounds in both flowers and leaves.
You want to cut the upper portion of the stems, collecting the open flowers, buds, and the top few sets of leaves. The lower stems and any woody growth at the base have little medicinal value.
Common Preparations and Which Parts They Use
All traditional and commercial preparations rely on the flowering tops, but the form varies:
- Capsules and tablets: Made from dried flowering tops that are extracted with alcohol or water, then concentrated and standardized. This is the form used in most clinical research on depression.
- St. John’s wort oil: Fresh flowering tops are soaked in a vegetable oil (traditionally olive or sunflower) and left in sunlight for two to six weeks. The oil turns a deep red as hypericin dissolves into it. This is used topically for skin irritation, minor wounds, and nerve pain.
- Tea: Dried flowers and leaves are steeped in hot water. This delivers a lower and less consistent dose of active compounds than standardized extracts.
- Tinctures: Fresh or dried flowering tops are soaked in alcohol for several weeks, producing a concentrated liquid extract.
The oil preparation is particularly interesting because the German Pharmacopoeia and Swiss Pharmacopoeia both include formal instructions for it. The German method calls for fresh or dried flowering tops macerated in vegetable oil at a 1:4 ratio. The Swiss version specifies olive or sunflower oil with two to three weeks of sun exposure. In Serbian folk medicine, the same process uses sunflower oil at a 1:5 ratio for 40 days.
A Note on Drug Interactions
The same compound that makes St. John’s wort effective for mood, hyperforin, also triggers a significant concern. Hyperforin activates a receptor in the liver that ramps up production of a key enzyme responsible for breaking down more than half of all prescription medications. This means taking St. John’s wort can cause your body to metabolize certain drugs faster than expected, reducing their effectiveness. Birth control pills, immunosuppressants, blood thinners, and many other medications are affected.
This interaction is directly tied to the flowering tops and their hyperforin content. It’s not a minor footnote: it’s one of the most well-documented herb-drug interactions in medicine, and it applies to every form of the plant, whether you’re taking capsules, tincture, or tea.