Green Tea

Anticoagulant, antiplatelet agents (examples include coumadin and aspirin):
Green tea contains Vitamin K and can make coumadin (warfarin) ineffective or less effective.
Green tea should not be taken if you are on aspirin because they both prevent platelets from clotting. The use of green and aspirin together may increase your risk of bleeding.

Beta-lactam Antibiotics (examples include cephalosporins and penicillins):
Green tea may increase the effectiveness of beta-lactam antibiotics by reducing bacterial resistance to treatment.

Benzodiazepines (used for anxiety; examples include valium and ativan):
Caffeine reduces the sedative effects of benzodiazepines.

Beta-blockers (examples include propranolol and metoprolol):
Caffeine can increase blood pressure in people taking beta blockers.

Clozapine (used as an antipsychotic medication):
Green tea can reduce the effectiveness of clozapine if green tea is taken less than 40 minutes before taking clozapine.

Ephedrine (used as bronchodilator, vasopressor, decongestant):
Green tea and ephedrine together will increase the incidence of agitation, tremors, insomnia, and weight loss.

Lithium (for treatment of mania and depression):
Green tea can reduce blood levels of lithium (a medication used to treat manic/depression).

Monoamine oxidase inhibitors (MAOIs) (used for treating depression; examples include phenelzine and tranylcypromine):
Green tea and MAOI's taken together may cause hypertensive crisis (severe increase in blood pressure).

Oral contraceptives:
Oral contraceptives and green tea will increase the effects of caffeine.

Phenylpropanolamine:
Green tea plus phenylpropanolamine cause mania and severe increase in blood pressure.