Herbal Drug Interactions

Herbal Drug Interactions

1. Scope, definitions & why herbal interactions matter

• A herbal drug interaction (a subset of drug–drug interaction) is a modification of the response to a conventional drug caused by concurrent intake of a herbal/botanical product or dietary supplement; the change is usually quantitative (effect increased or decreased) but may be qualitative (a new, abnormal response) (KDT 8e Ch.71, p.987).
• Regulatory/legal status drives the risk. In the USA, dietary supplements (DS) — including herbal/botanical "medications" — are regulated under the Dietary Supplement Health and Education Act (DSHEA, 1994) as foods, not drugs; they reach market with no pre-marketing proof of efficacy or safety, and the FDA must prove a product harmful before restricting it (Katzung 16e Ch.65, pp.1203–4; Harrison 22e Ch.495, p.3926).
• In India the parallel regulator is the DCGI, which (like the FDA) can withdraw a supplement judged unsafe but does not pre-clear it; herbal products do not undergo strict clinical trials (Padmaja 7e Ch.62, p.729).
• Magnitude of use. WHO estimates ~80% of the population of developing countries uses plants as medicines; US DS sales exceeded $32 billion (2021) (Katzung 16e Ch.65, p.1203). In the 2012 US NHIS, non-vitamin/non-mineral DS were used by ~18% of adults and 5% of children (Harrison 22e Ch.495, p.3925).
• The four structural reasons herbal interactions are dangerous and under-recognised:
  • Unstandardised, variable constituents — a botanical may contain hundreds of active and inactive constituents; relative amounts vary with species, plant part, growing conditions and extraction method, and most commercial products are not standardised to any one constituent. Inadequate interaction testing is a key risk (Katzung 16e Ch.65, pp.1205–6; Harrison 22e Ch.495, p.3926).
  • Misidentification, adulteration and contamination — DNA barcoding of 44 North-American herbal products found product substitution in 32% of samples (Newmaster 2013); products may be adulterated with pharmaceuticals (steroids, diuretics, stimulants, PDE-5 inhibitors), heavy metals (mercury, lead, arsenic) or aflatoxin from Aspergillus flavus on improperly stored material (Katzung 16e Ch.65, p.1204; Padmaja 7e Ch.62, p.730).
  • Under-reporting — consumers rarely report adverse events, and chemical analysis of the implicated product is rarely done, so cause (primary ingredient vs adulterant) is usually unknown (Katzung 16e Ch.65, p.1204).
  • Patients do not disclose — because DS are labelled "natural" and assumed safe, patients often do not tell their physician they take them; maintaining a non-judgemental, open attitude increases disclosure (Katzung 16e Ch.65, p.1204; Harrison 22e Ch.495, p.3925).
• "Natural ≠ safe." Botanicals may be inherently inert or toxic; misidentified mushrooms cause liver failure, contaminated tryptophan caused eosinophilia–myalgia syndrome, and Aristolochia species cause genitourinary malignancy and interstitial nephritis (Katzung 16e Ch.65, p.1204; Harrison 22e Ch.495, p.3926).
• Where the danger concentrates. Herb-drug interactions matter most with narrow-therapeutic-index (NTI) object drugs — anticoagulants, antiseizure drugs, antiretrovirals, immunosuppressants (calcineurin inhibitors), digoxin and cancer chemotherapeutics — because a small change in exposure crosses the efficacy/toxicity threshold (Harrison 22e Ch.495, p.3926; KDT 8e Ch.71, p.988).
• Concentrate vs food matters: natural products ingested as food or herbal teas, rather than concentrated extracts, are less likely to cause harm — dose and formulation drive the interaction (Harrison 22e Ch.495, p.3926).