If used excessively (or for very long periods of time), they can cause some hepatic (liver) and/or renal (kidney) damage. 

The hepatic damage occurred to me not long after my diagnosis as a Type 2 diabetic.  The renal damage occurred to my then-girlfriend's daughter when she tried to diet off 190 pounds.

I have always had good results with amaryllis and/or clivia.  Planting the two side-b-side in one corner of my yard has resulted in an unbelievably large number of  hummingbirds who sort of  "hang out" there.