Quick, what do you notice in the following?
“Waiter, I’ll have grapefruit juice. A friend says it’s okay for dieting.” - Aida
“I’m new in this networking business. Our ginseng liquid extract has been fantastic and I’m drinking some.” - Lorna
 “I’ve been on garlic capsules for three weeks now.” - Fe
Those lines are from some of my patients (not their true names) on different occasions. Now you’d say they mentioned herbal products in passing. And you’d say the herbals are natural and innocuous enough. They shouldn’t be a cause for alarm. Or wait, should they?
You see, for all the vogue of hugging trees and going green, organic and natural nowadays (and the multimillion industry arising from such), the clueless layperson can be prone to being exploited by unscrupulous people peddling gooey herbal stuff and all. Therefore, it pays to expose the common dangerous lie once and for all about herbal medicine: anything that is natural is said to be safe for our health. Nonsense.

Consider the case of Aida, Lorna and Fe. All are in their early 50s. The herbals they mentioned seemed harmless until I tell you that Aida and company have high blood pressure, high cholesterol and heart disease. They are taking prescription meds for such. My spidey sense would say Aida should never drink grapefruit while on medication. Grapefruit may help increase the effect of her meds to a dangerous level (for example, very low blood pressure) (Maskalyk 2002).
Second, Lorna should have known that despite claims about ginseng being cure-all, the side effects of some types of ginseng may include palpitations and elevated blood pressure, which may help diminish the effects of her meds (Tachjian A et al 2010).
Meanwhile, Fe is on aspirin for the prevention of heart attack and stroke. It would be prudent to know that both ginseng and garlic may possibly increase the risk of bleeding (Heck et al, 2000), especially in those who are on anti-clotting meds such as aspirin.
As a doctor, I acknowledge the potential of herbal medicines. In fact, many mainstream drugs today started out as herbals. Herbals in themselves do not constitute a magic potion. They're more like an indication of a holistic mindset about healthy lifestyle; a complement to the more mainstream regimens. Now, in the face of the chronic shortage and high cost of healthcare delivery in the country, herbal medicines are tantalizing linchpins in responsible self-care: they are inexpensive. They can be had regardless of a doctor’s prescription. Most if not all don’t require expert consultation. They are easily available everywhere. If you are enterprising enough, you can grow and concoct your own. And lastly, most (read: not all) are actually harmless in many situations – which is way better than hastily generalizing that everything natural is totally okay.
Now, what can be deemed from the advantages? Paradoxically, these advantages lead to disadvantages. Observe: because they’re readily available everywhere and are loosely regulated by the FDA if at all, the problems are legion.
The main problem is it’s difficult to assess their quality. Many entities making and selling herbal meds lack: a) scientific data (on mixing herbs and drugs, uniform dosaging, clinical efficacy, safety, toxicology, heavy metal content, and microbial contamination), b) standardization and quality control in production, and c) marketing techniques that don’t make outrageous claims or fool people. For example, if someone sells you Chinese marine herbal meds, how can you be sure they’re not contaminated with lead or mercury, and not adulterated with starch? Worse, there are no formal tests that can tell apart quality herbs from bogus and inferior ones.
The other related main problem is that many herbals today are taken: a) unevenly; b) in mega doses; c) in illnesses that are worsened by such; d) with mainstream heart and/or non-heart medications; e) long-term; f) and without the supervision of a duly recognized health professional. As such, the resulting interactions with mainstream western heart medicines may prove harmful. The herbals may potentiate the drugs, render them less effective, lead to new side effects, affect medical tests, or impact the disease in themselves.

Recently, a group of doctors in the US (Tachjian et al 2010) did a study on commonly used herbs and how these interact with heart medicines. Below is a table condensed from their study. It tabulates some herbs to avoid if one has heart/cardiovascular disease or is taking meds for such.

[Herbs - Heart (e.g., cardiovascular) medicines - Standalone effect of the herb on heart/cardiovascular system; effect of the herb on the medicine]
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Alfalfa - warfarin - ↑ bleeding
Aloe vera -Digoxin - ↓ blood potassium causing digitalis toxicity and palpitation
Angelica (dong quai) - Warfarin - ↑ bleeding
Bilberry - Warfarin - ↑ bleeding
Butcher's broom - Alpha-blockers - ↓
Danshen - Anticoagulant or antiplatelet agents - ↑ bleeding
             - Digoxin - ↑
Echinacea - Amiodarone or ibutilide - Palpitation
               -  Statins, fibrates, niacin - ↑risk of liver toxic effects
Ephedra - Class IA and class III antiarrhythmics (types of meds for palpiations) - Palpitation
             - Beta-blockers - ↓leading to hypertension and tachycardia
Fenugreek - hypoglycemia
                - Warfarin - ↑bleeding risk
Fumitory - Beta-blockers, calcium-channel blockers, cardiac glycosides (digoxin) - ↑
Garlic - Aspirin, clopidogrel, warfarin, or heparin-like drugs - ↑ bleeding risk
Ginger - Warfarin - ↑ bleeding risk
Ginkgo biloba - Potential risk of seizures
                    - Aspirin; warfarin - ↑ bleeding risk
Ginseng - ↑ blood pressure; palpitation
    - Digoxin - interferes with digoxin blood test, leading to falsely increased levels
    - Warfarin - ↓
Gossypol - diuretics; ↑ diuretic effect; ↓ low blood potassium
Grapefruit juice - statins, irbesartan, calcium-channel blockers; drugs for palpitation, warfarin - ↑
Green tea - Warfarin - ↓ (contains vitamin K)
Hawthorn - Digoxin; calcium-channel blockers or nitrates - ↑
Irish moss- Meds for high blood pressure - ↑
Kelp- Antihypertensive; anticoagulant agents - ↑
Khella - Anticoagulant agents and calcium-channel blockers - ↑
Licorice - ↑ blood pressure; ↓ blood potassium
            - Spironolactone; digoxin - ↑
Lily of the valley - Beta-blockers, calcium-channel blockers, digitalis, quinidine - ↑
Ma-huang (ephedra) - ↑ heart rate and blood pressure
Night-blooming cereus - Angiotensin-converting enzyme inhibitors, antiarrhythmics, beta-blockers,
                                    calcium-channel blockers, cardiac glycosides - ↑; palpitations
Oleander - Heart block; ↓ blood potassium; palpitation; death
Saw palmetto - Anticoagulant or antiplatelet agents - ↑ bleeding
Soy milk -Warfarin -↓
St. John's wort - Digoxin - ↓
      - Clopidogrel - ↑ bleeding
      - Warfarin - ↓
      - Simvastatin - ↓
      - Class IA and III antiarrhythmic agents (types of meds for palpiations) - ↓ (resulting in palpitations)
Storphanthus - Cardiac glycosides (digoxin) - ↑
Yohimbine - Clonidine, Guanabenz - ↓ blood pressure reduction effect of centrally active agents
                - ACE inhibitors (“-april” drugs) - ↓; hypertension
- Beta-blockers ( “-olol”meds) - ↓; hypertension; increases heart rate
______________________________________________________________________
Note that the list of side effects and interactions above is not exhaustive (it doesn't even mention the effects of animal products used in complementary medicine). This may change if new insights and breakthroughs arise in the future. Hence, they are better properly discussed with one’s healthcare provider.
Here are final tips. Always inform your healthcare provider if you are taking both heart medicines and herbals. If you are still in the stage of planning to take both, stop and ask: how would the herbal preparation affect the prescription medicine and my health? Consult. Any unexpected effect that you sense after taking the herbal should be reported to your healthcare provider as soon as possible.
Sources
Heck A M, DeWitt BA & Luke AL. Potential interaction between alternative medicine and Warfarin. Am J Health Syst Pharm 2000; 57:1221-1227.
Maskalyk J. Grapefruit juice: potential drug interactions. Canadian Med Assoc Journ 2002;167(3):279-280.
Tachjian A et al. Use of Herbal Products and Potential Interactions in Patients with Cardiovascular Diseases. JACC 2010;55(6):515–525.