Harmful to any form of life and/or not found in nature - beware!
If we don't succeed in becoming the only species to poison ourselves off the planet, there are going to be a lot of "I told you so's" in the coming years. The evidence should already be overwhelming to anyone who does not have their eyes and minds closed. Increased toxic loads, lower sperm counts, lower immune functions, increased mental disorders, increased cancer and chronic illness in all age groups . . . what is it going to take?
I have long maintained that if something is deadly or harmful to one form of life it is likely to be deadly or harmful to other forms of life, especially if it is something not found in nature. The harm may not be as readily apparent, but it is there and it is likely all the more harmful in combination with all the many pesticides, carcinogens/mutagens and other toxins we have introduced to our environment and our bodies.
Another rule I believe in is that if something is not found in nature it is probably not a good idea to injest, inject or otherwise place it into our natural bodies. That includes not only lab-created drugs and synthetics, but also unique isolates not found in nature without supporting compounds. It also, in my very strong opinion, includes the single and cumulative affects of the hugely increased schedule of vaccines which have had virtually no long term studies for safety individually or cumulatively.
Those rules may not be absolute truisms, but they are close enough for me.