Ground cinnamon is very safe, the volatile oils can however cause a skin rash. Small amounts of coumarin can be found in Cassia and other cinnamons, generally only large doses of this compound will cause blood-thinning and liver problems, but it's something to be aware of. Also if you're planning on having surgery you should stop the use of cinnamon at least one week before going in as it has a blood thinning effect. You should also take care to monitor your blood sugar to avoid an unsafe drop in blood pressure.
The nettle plant can actually be used in a variety of ways, teas, capsules, tinctures, and extracts are all great ways to get the benefits of the nettle. Capsules can be found and used to help manage hay fever symptoms, anywhere from 300 to 800 mg is generally the recommended dosage. Teas are often consumed to gain the strong diuretic effects that nettles have, because of this diuretic effect it has been used for things such as arthritis, prostate health, and high blood pressure.