Herbal	Possible Food/Drug Interactions
MAOIs = monoamine oxidase inhibitors.
Black cohosh	May further reduce lipids or blood pressure when combined with prescription medications; may increase antiproliferative effect obtained with tamoxifen.
Chamomile	May increase bleeding when used with anticoagulants; may increase sedative effect of benzodiazepines.
Dong quai	May increase effects of warfarin.
Echinacea	May interfere with immunosuppressive therapy.
Garlic	May increase bleeding time with aspirin, dipyridamole, and warfarin; may increase effects and adverse effects of hyperglycemic agents.
Ginkgo biloba	May increase bleeding time with aspirin, dipyridamole, and warfarin; may increase blood pressure when used with thiazide diuretics.
Ginseng	May adversely affect platelet adhesiveness/blood coagulation; may increase hypoglycemia with insulin; may interfere with antipsychotic drugs; may cause hypertension when used long-term with caffeine.
Kava kava	May increase central nervous system depression when used with alcohol and sedatives; may cause hepatotoxicity.[12]
St. John's wort	May cause serotonin syndrome when used with antidepressants and drugs using p450 microsomal enzyme for metabolism.[13] Interacts with procarbazine.
Ma huang (ephedra)	Increases toxicity with beta-blockers, MAOIs, caffeine, and St. John’s wort.
Yohimbe	Decreases effect of antidepressants, antihypertensives, hyperglycemic agents, MAOIs, and St. John’s wort.

References

12. U.S. Food and Drug Administration.: Consumer Advisory: Kava-Containing Dietary Supplements May Be Associated with Severe Liver Injury. Washington, DC: U.S. Food and Drug Administration, 2002. Available online. Last accessed August 11, 2009. 
    
    
13. Piscitelli SC, Burstein AH, Chaitt D, et al.: Indinavir concentrations and St John's wort. Lancet 355 (9203): 547-8, 2000.  [PUBMED Abstract]