Herbs and herbal formulas should always be used with caution, and common sense.
A common misconception about herbalism and the use of ‘natural’ products in general, is that ‘natural’ equals safe. However many plants have chemical defense mechanisms against predators that can have adverse or lethal effects on humans. Examples are poison hemlock and nightshade, which can be deadly. Herbs can also have undesirable side-effects just as pharmaceutical products can. These problems are exacerbated by lack of control over dosage and purity.
Furthermore, if given in conjunction with drugs, there is danger of ‘summation’, where the herb and the drug have similar actions and add together to make an ‘overdose’. In animals, there are other dangers. There may be residues in food from farm animals (e.g. eggs, milk, meat) or danger of ‘doping’ in competition animals. The latter may also apply to human athletes.
The common names of herbs may be shared with others with different effects. For example, in one case in Belgium in a TCM-remedy for losing weight, one herb was swapped for another resulting in kidney damage. One variety of the herb causes elevated blood pressure and increased heart rate, versus another variety for the weight-loss remedy, the varieties are differentiated by the suffix in the Latin names.
The legal status of herbal ingredients varies by country. For example, Ayurvedic herbal products may contain levels of heavy metals that are considered unsafe in the U.S., but heavy metals are considered therapeutic in Ayurvedic medicine.
Those wishing to use herbal remedies should first consult with a physician, as some herbal remedies have the potential to cause adverse drug interactions when used in combination with various prescription and over-the-counter pharmaceuticals. Dangerously low blood pressure may result from the combination of an herbal remedy that lowers blood pressure together with prescription medicine that has the same effect.