St. John’s Wort (Hypericum perforatum) is one of the most widely used herbal supplements globally, taken primarily for mild-to-moderate depression and anxiety. It is one of the few herbal supplements with a well-established, clinically significant photosensitivity risk — and it causes photosensitivity through a unique mechanism not shared by most pharmaceutical drugs.

Why St. John’s Wort Causes Photosensitivity

The primary photosensitizing compound in St. John’s Wort is hypericin — a naphthodianthrone pigment that gives the plant its characteristic red oil (visible when crushing the flower petals). Hypericin is a potent photosensitizer:

Hypericin absorbs UV-A and visible light (peak absorption ~590nm, in the visible red spectrum)
The excited hypericin molecule generates singlet oxygen and reactive oxygen species
These reactive species cause oxidative damage to skin cells and potentially retinal cells

Unlike most drug photosensitizers that cause phototoxicity at UV-A wavelengths only, hypericin’s absorption of visible light means that even indoor lighting and visible daylight can trigger reactions in highly exposed individuals — not just direct UV sunlight.

Clinical Presentation

St. John’s Wort photosensitivity causes:

Erythema, burning, and edema in sun-exposed skin
Blistering and vesiculation in severe cases (more common with very high doses)
Predominantly affects light-skinned individuals
Generally mild at typical supplement doses (300mg standardized extract 3×/day)
More severe reactions documented in livestock grazing on Hypericum plants in full sunlight — historically the first recognized hypericin toxicity

Clinical Risk Assessment

At standard supplement doses (900mg/day total), clinically significant photosensitivity is relatively uncommon but does occur. Risk is higher with:

High doses or concentrated extracts
Fair skin (phototype I–II)
Intense sun exposure (outdoor work, beach, high altitude)
Concurrent use of other photosensitizing medications (additive risk)

Drug Interactions (Critical)

Beyond photosensitivity, St. John’s Wort induces CYP3A4 — a major liver enzyme. This causes clinically significant interactions with many medications, reducing their blood levels:

Oral contraceptives (pregnancy risk)
Antiretrovirals (HIV treatment failure)
Cyclosporine (transplant rejection)
Warfarin (thrombosis risk)
Many other drugs

Patients and clinicians must screen for drug interactions before starting St. John’s Wort regardless of the photosensitivity concern.

Prevention and Management

Apply SPF 30–50+ broad-spectrum sunscreen before sun exposure
Protective clothing and hat for prolonged outdoor activities
No special management needed beyond sun protection at standard doses
If significant photosensitivity occurs: reduce dose or discontinue; reactions typically resolve within days to weeks
Patients with history of severe phototoxic reactions or on multiple photosensitizing drugs should consider pharmaceutical antidepressants instead

Sources

Brockmoller J, et al. “Hypericin and pseudohypericin: pharmacokinetics and effects on photosensitivity in humans.” Pharmacopsychiatry. 1997;30(Suppl 2):94-101.
Schempp CM, et al. “Topical application of St. John’s Wort.” Phytomedicine. 2000.
Linde K, et al. “St John’s wort for depression.” Cochrane Database Syst Rev. 2008.

St and Photosensitivity: Side Effects & Sun Protection