The growing conditions (adherence to good agricultural
practice, limitation of pesticide usage) and storage conditions
(humidity, temperature) are also important as safety criteria. In
complex combinations of PFS, herbs may interact with each
other, or with other drugs or nutrients. Herbal supplements can,
like drugs, cause medical problems if not used correctly or if
taken in large amounts. Even then, some consumers can experience
negative effects even when they follow the instructions on
the PFS label. Allergic reactions may occur, which are hard to
predict because they are idiosyncratic.