It is generally assumed that herbal remedies are safer than conventional synthetic drugs. There is some truth in this, as most of the herbs used in practice do have a safer profile than even the mildest synthetic drug.
However, complacency is unwise as ther have been reported infrequent but occasionally severe reactions to herbal medicines - and this is why it is important to get your herbal prescription from a fully trained and qualified herbalist.
Pharmacological activity of constituents.
Herbs contain compounds that may exert a direct pharmacological effect, in the same way as drugs do - and if these effects are not what is required for the presenting complaint then adverse reactions may result.
Intrinsically toxic components.
For example, some herbs contain alkaloids which have a direct effect on the central nervous system. In fact many illegal drugs, such as heroin and cocaine, are derived from herbal alkaloids - just to show how strong their effects can be! Whilst the alkaloids in this example are not necessarilly toxic, their misapplication by giving the wrong dose etc. can be quite harmful.
Chronic or acute overdose.
Again, knowing the safety margins for dosage of herbal remedies is important.
Interactions between herbs.
Some herbal compounds can cancel each other out, whilst others can be synergistic and produce additional effects to those that would come from just using the individual herbs. It is important for a herbal practitioner to have the experience and knowledge to choose herbs that combine safely.
Herb / drug interactions.
At your consultation you will always be asked if you are already on any sort of medication. Some herbs can interact with pharmaceutical drugs, although this is often exaggerated by the medical profession. Herbalists are the people most likely to know whether or not a herb is safe to use with a particular medicine, as it is obviously an important part of a herbalist's education - whereas it is unlikely that the average medical student would spend more than half an hour in their entire training considering this issue. See here for more details.
Patient susceptibility (such as allergies).
Occasionally we discover that a patient might be allergic to a particular herb - and if you already know that you have an allergy to a particular plant family it would be very wise to let the herbalist know this at your first meeting! Also, some people respond differently to others to particular herbs. A good example of this would be Valerian - which is usually regarded as a sedative, helping people to relax and get to sleep more easily, but in some people can have the opposite effect.