Here is a good rule of thumb - if you can or could or do eat it in a NORMAL diet - then it is not harmful. This applies to things like oregano, thyme, garlic, onions, sage, cinnamon, ginger, cloves, nutmeg, tumeric, peppers, and so forth and so on.
Things that are medicinal - ones that are not eaten as part of a regular diet - should be used AS medicine - and with caution. Wormwood, black walnut, hyssop, gentian, pau d' arco and so forth.