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Many patients in India take Ayurvedic medicines alongside conventional drugs, often without telling either prescriber. Herb–drug interactions are therefore a practical concern for integrative care, yet the evidence is scattered across pharmacology, clinical pharmacology and case literature. This critical review summarises human evidence for interactions involving commonly used Ayurvedic drugs, examines their mechanisms, and relates them to the classical Dravyaguna concepts of samyoga (combination), yogavahi (a carrier that enhances other substances) and viruddha (incompatibility). Human pharmacokinetic or clinical evidence was found for seven drugs. Piperine, the principal alkaloid of Pippali and Maricha, increased exposure to phenytoin, propranolol and theophylline in volunteers, consistent with its inhibition of CYP3A4 and P-glycoprotein. Gugulipid reduced peak concentrations and exposure of propranolol and diltiazem, and guggulsterone activates the pregnane X receptor, which induces CYP3A. Garlic supplements halved saquinavir exposure. Two studies of curcumin with talinolol reported opposite effects. Case reports describe altered response to phenytoin with Shankhapushpi, to chlorpropamide with karela, and to warfarin with a fenugreek-containing product. For Ashwagandha, in vitro inhibition of major cytochrome P450 enzymes was weak, but its effect on thyroid hormones and rare liver injury raise pharmacodynamic concerns. Most evidence is from small, short studies of single compounds rather than classical formulations. We propose practical guidance for drugs with a narrow therapeutic index and a research agenda centred on realistic doses of classical preparations.