Pharmacognosy and Natural Products: Sources, Classes and Interactions
Classification
Natural products are organised by biosynthetic class: alkaloids (nitrogen-containing, often basic - morphine from Papaver somniferum, quinine from Cinchona, atropine from Atropa belladonna, vincristine from Catharanthus roseus, caffeine), glycosides (cardiac glycosides such as digoxin from Digitalis lanata; sennosides as laxatives), terpenes and terpenoids (artemisinin from Artemisia annua, paclitaxel originally from Taxus brevifolia), flavonoids/phenolics and polysaccharides (gums, mucilages). Knowing the plant family or source is a common exam item; knowing the class helps you predict solubility, stability and toxicity.
Standardisation and quality
Crude herb potency varies with species, plant part, harvest and extraction. Standardisation to a marker compound, correct botanical identity, and testing for adulterants, heavy metals, pesticides and microbial contamination are the quality controls. A "natural" label is not a safety claim.
Interactions are the highest-yield trap
- St John's wort - potent enzyme inducer; reduces effectiveness of oral contraceptives, warfarin, ciclosporin, some antiretrovirals; serotonergic with SSRIs.
- Ginkgo, garlic, ginseng - bleeding risk with anticoagulants and antiplatelets.
- Grapefruit juice - CYP3A4 inhibitor, raises many drug levels.
- Liquorice - pseudoaldosteronism: hypertension and hypokalaemia.
- Senna and other laxatives - electrolyte loss, interaction with digoxin toxicity risk.
Traps
Assuming standardised extracts are bioequivalent to a drug; forgetting that patients rarely volunteer herbal use, so you must ask; confusing source plants with similar names.
Worked example
A patient on warfarin starts St John's wort: induction increases warfarin metabolism, INR falls and thrombosis risk rises; stopping it later can overshoot the INR. Advise against it and monitor INR closely if any change occurs.