If you have high blood pressure, don't take an herb such as ephedra that elevates blood pressure. If you are a bundle of nerves from drinking too much coffee, don't take an herb such as green tea that is loaded with caffeine. But, always speak to your doctor or pharmacist. Those drugs are tricky and dangerous. Culinary herbs which should be used only for flavouring in small doses include sage, thyme, basil, savory, majoram, and cinnamon.
Blue cohosh is contraindicated except during and just before labour.
One nasty rumour going around is that there are no government regulations on herbs. I haven't found one. It was passed thanks to a tremendous grass-roots campaign to keep herbs on the market. In other words, a manufacturer may sell a product such as echinacea, which is useful against colds and flus, so long as the package doesn't say it will cure colds and flu. This is why you'll need to read books, articles and ask questions of someone knowledgeable to find out what herb, when and how you should take it.