In using herbs that invigorate blood and resolve blood stagnation, it is necessary to add herbs that promote Qi circulation to enhance therapeutic effects. Usually, practitioners will prescribe a mixture of different herbs. These herbal mixtures are based on tried and tested formulae from medical books that date back over hundreds of years. Every herb has its own characteristics and effects. Combining different herbs helps to increase their potency and minimize any unwanted consequences. The herbs work together to achieve maximum benefit for patients. Herbs can come in a dried form, or as herbal capsules, tablets, ointments and creams. Dry herbs are available only by prescription from a practitioner. They are more powerful than capsules or tablets, but are often more expensive. They are also more time-consuming to use, because they need to be brewed into drinks. Most herbs are derived from roots, tubers or tree bark and the stems of plants, flowers or seeds.