Artimisia annua (Sweet Annie)
Don't take if you have ulcers or gastrointestinal disorders, or if you're  taking antacids, sucralfate, proton pump inhibitors, and histamine-receptor antagonists because it increases the production of stomach acid.  Artemisia can induce seizures making antiseizure medications less effective.

Ephedra   
Caffeine, decongestants, stimulants
Don't use ephedrine-containing supplements if they have cardiovascular disease, hyperthyroidism, diabetes mellitus, benign prostatic hypertrophy or glaucoma.

Bilberry
Anticoagulants / Antiplatelets: may increase  the risk of bleeding if used concurrently due to platelet aggregation inhibition and prostaglandin modulation.

Alfalfa
Increases in rate of metabolism of xenobiotics in the liver. May reduce the effect of warfarin. Because of its estrogenic activity, high levels of alfalfa may interfere with contraceptives and hormonal therapy. Alfalfa may increase effects of diuretic medication. Alfalfa supplements may interfere with hypoglycemic agents.

Ashwagandha
May increase the sedative effect of barbiturates.

Black cohosh
Don't take with Tamoxifen itmay have an additive antiproliferative effect. With Chemotherapy drugs it may increase the toxicity of doxorubicin and docetaxel. Black cohosh may interact with drugs that are metabolized by CYP3A4 enzyme.

Borage
Oil may lower the seizure threshold due to its gamma linoleic acid content, when used with Phenothiazines, Tricyclic antidepressants: Theoretically, may lower seizure threshold due to gamma linoleic acid content. Seizures have been documented with evening primrose oil, but not borage oil. NSAIDS interfere with the synthesis of prostaglandin E. and  decrease the effects of borage oil.

Cactus-Avoid cactus if you're taking digoxin or other heart medications, unless you're under the care of a doctor and herbalist.

Cascara Sagrada
Can cause excessive loss of potassium with diuretics. Used with Digoxin may potentiate cardiac effects.

Cats Claw
May have an additive or synergistic hypotensive effect with Antihypertensives. May have an additive anticoagulant effect with Anticoagulants / Antiplatelets.
In-vitro, cat's claw inhibits CYP3A4, may increase the serum levels of drugs such as protease inhibitors, nonnucleoside reverse- transcriptase inhibitors, cyclosporine, some benzodiazepines and others.

Chamomile
Anticoagulants / Antiplatelets (warfarin/Coumadin, ect.) may increase the  anticoagulant effects and inhibit platelets due to coumarin content.

Chasteberry
Don't take if you're taking hormonal medications, birth control pills, it may interfere with their effects.Don't take if you're taking dopamine D2-antagonists, such as chlorpromazine, haloperidol, or prochlorperazine.

Chrysanthemum may have an additive hypotensive effect.

Dandilion
Toxicity to lithium may be worsened due to sodium depletion . May have additive effects due to diuretic activity.  Hypoglycemics: may have additive effects due to hypoglycemic activity.

Devils Claw
May reduce efficacy due to increased production of stomach acid if taken with Antacids / H2 Antagonists. With Beta blockers / Digoxin: Devil's claw may cause bradycardia and weaken heart contractions and coronary blood flow. May also have additive anticoagulant activity.

Echinacea
May increase the blood-thinning ability of Coumadin, also known as Warfarin. Echinacea can cause liver toxicity. It should be avoided in combination with other medications that can affect the liver (such as ketaconazole, leflunomide (Arava), methotrexate (Rheumatrex), isoniazide (Nizoral).

Fenugreek
Anticoagulants & Fenugreek  have effects due to coumarin content. With Antidiabetic agents: possible enhanced glucose-lowering effects. Insulin dosage may have to be adjusted with regular use of fenugreek. Absorption of other oral medications may be impaired due to high mucilaginous fiber content of fenugreek seeds. MAOIs & Fenugreek may potentiate their effect due to amine content.

Feverfew
Believed to be the natural remedy for migraine headaches, should never be taken with Imitrex or other migraine medications. It can result in the patient's heart rate and blood pressure to rise dangerous levels. May have additive effect if used with Anticoagulants / Antiplatelets.

Flax seed- can inhibit the absorption of many drugs. Wait at least an hour before you take theiis bowell tonic herbs. High mucilage content can also cause a change in your blood sugar, so be careful with them if you're a diabetic.

Garlic
May decrease the effectiveness of several antihypertension drugs, as well as birth control pills.
Garlic capsules combined with diabetes medication can cause a dangerous decrease in blood sugars. Some people who are sensitive to garlic may experience heartburn and flatulence. Garlic has anti-clotting properties. Garlic might speed how the body breaks down estrogen found in some birth control pills  and may decrease their effectiveness.
 
Germander
May have additive toxicity with other drugs known to cause renal (e.g. aminoglycosides), hepatic, neurotoxicity (e.g.taxanes), or myelosuppression.

Ginkgo biloba   
Has potential interactions with garlic, vitamin E and medications with antiplatelet or anticoagulant properties. Aspirin, warfarin (Coumadin), ticlopidine (Ticlid), clopidogrel (Plavix), dipyridamole (Persantine),

Ginseng-
With Warfarin can increase blood pressure, making it dangerous for those trying to keep their blood pressure under control. Should never be taken with the blood-thinning drug, Coumadin. In rare cases, ginseng may overstimulate resulting in insomnia. Consuming caffeine with ginseng increases the risk of overstimulation and gastrointestinal upset.
Hexobarbital: Eleuthero inhibits metabolism possibly by inhibition of cytochrome p450 2C19.
Digoxin: Elevate serum digoxin levels.
Monomycin / Kanamycin: Increases efficacy of these antibiotics due to enhanced T-lymphocyte activity.

Siberian Genseng
And Insulin / hypoglycemics: Theoretical additive hypoglycemic effect.

Goldenseal
Active ingredient will raise blood pressure, interfering antihypertensive medications, especially beta-blockers. For patients taking medication to control diabetes or kidney disease, this herb can cause dangerous electrolyte imbalance. High amount of consumption can lead to gastrointestinal distress and possible nervous system effects. Not recommended for pregnant or lactating women.

Gotu Kola
Theoretically gotu kola may interfere with blood glucose levels when taking Hypoglycemics / Insulin.Gotu kola may interfere with cholesterol lowering agents.

Guarana
If you are taking any medication that advises you against taking any drink with caffeine, you should avoid taking this stimulant. It may cause insomnia, trembling, anxiety, palpitations, urinary frequency, and hyperactivity. Avoid during pregnancy and lactation period. Long term use of Guarana may lead to decreased fertility, cardiovascular disease, and several forms of cancer.

Guggul
May increase the effects of aspirin, NSAIDs and warfarin.  May have thyroid stimulating activities.
Guggul can induce CYP3A4 activity and may interact with substances metabolized by the same enzyme.

Hawthorn
May be effective in reducing angina attacks by lowering blood pressure and cholesterol levels, should never be taken with Lanoxin (digoxin), for heart ailments. The mix can lower your heart rate too much, causing blood to pool, bringing on possible heart failure.

Kava
Sedatives, sleeping pills, antipsychotics, alcohol.  Kava  has antianxiety, pain relieving, muscle relaxing and anticonvulsant effects, should not be taken together with substances that also act on the central nervous system, such as alcohol, barbiturates, anti depressants, and antipsychotic drugs.

Lavender
May increase or potentiate the CNS depressant effects of sedative or hypnotics drugs.

Licorice
Many Chinese ready-made supplements as well as herbal tea formulas that you make, contain licorice. Don't use licorice or formulas that contain it when taking diuretics like furosemide (Lasix) because licorice can cause potassium depletion in your body, especially when you use the herb for more than ten days or so.

Marsh-mallow
can inhibit the absorption of many drugs. Wait at least an hour before you take this bowel-tonic herb. Herbs with a high mucilage content can also cause a change in your blood sugar, so be careful with them if you're a diabetic.

Myrrh
With Diabetes therapy: Interaction with antidiabetic therapy is possible as hypoglycemic properties have been documented.

Nettle
Don't use if you're taking diclofenac or if you're taking diuretics, nettle can have additive effects.

Passion Flowe
May potentiate the effects of pentobarbital. May increase the sedative effects of benzodiazepines. May have additive anticoagulant effect. May increase the sedative effects of alcohol.

Pau d'arco
May potentiate effects of Anticoagulants /  Antiplatelets.

Psyllium
can inhibit the absorption of many drugs. Wait at least an hour before you take this bowel-tonic herbs. Herbs with a high mucilage content can also cause a change in your blood sugar, so be careful with them if you're a diabetic.

Red clover
Can increase the risk of bleeding, don't take  if you're taking warfarin or other blood thinners.  Red clover may have estrogenic activity and should be avoided if you've got hormone-sensitive diseases such as estrogen receptor-positive breast cancer, ovarian cancer, endometriosis, etc.

Rhubarb
Chronic Use can cause potassium loss due to stimulant laxative effect, increase potential risk for hypokalemia.  Don't use with Digoxin.

Salvia divinorum (Diviner's sage)
May alter the analgesic effects of other opioids.

Sassafras
Can have an additive effect if you're taking barbiturates such as phenobarbital .

Stevia
With Verapamil: may have additive antihypertensive effects. With Diabetes drugs: it may affect blood glucose level, so cautious use is advised.

St. John's wort
Antidepressants & or over-the-counter diet medications can cause blood pressure to shoot up. Should be avoided in combination with other medications that can affect sun sensitivity (such as tetracycline/Achromycin, sulfa- containing medications, piroxicam (Feldend). St. John's wort can also cause headaches, dizziness, sweating, and agitation when used in combination with serotonin reuptake inhibitor medications such as fluoxetine (Prozac) and paroxetine (Paxil).

Yerba Mate
Effects of aspirin, acetaminophen, diuretics,CNS Stimulants is increased due to caffeine in mate. May increase effects of Theophylline & lead to possible toxicity.  Effects of benzodiazepines are reduced by caffeine content in mate. May alter effects of Clozapine. Mate can cause hypertensive crisis if used with  Monoamine-oxidase inhibitors (MAO-I).

Yohimbe
Can cause hypertension when used with antidepressants. MAO inhibitor for depression: don't take the African aphrodisiac herb yohimbe. Toxic effects of yohimbine result when used with phenothiazines such as chlorpromazine and promazine. Drugs that are CNS stimulants, such as epinephrine, ephedrine, amphetamines and cocaine, can potentiate the toxic effects of yohimbine. The toxicity of yohimbine may be increased by tranylcypromine and phenelzine. Yohimbine acts synergistically with naloxone resulting in increased anxiety, tremors, palpitations, as well as erections in male subjects.