it used to be in the Eupatorium genus [Eupatorium ageratoides] but has recently been re classified as Ageratina altissima. it is very pretty for the back of a border.
do you have it in your garden? or is this a photo of it in the wild?

I have seen this grown in mixed herbaceous borders and although it does have a nasty toxin called tremetol, it is a stunning plant. Livestock that eat it get tremors and the toxin is incorporated into the tissues of the animal and in the milk. So if you eat the meat and drink the milk you then suffer from poisoning. It accounted for a lot of deaths among the early settlers in the USA. Their cattle ate it and they didn't know it was a problem.

White snakeroot, Ageratina altissima (or Eupatorium) is not the problem it once was. Unusually, its toxins are not all broken down by the first creature to consume it and some get passed into the animal's milk.

Human deaths, of which there are thought to have been many thousands in the early days of European settlement in the USA, generally arose from drinking toxic cows' milk rather than direct ingestion because, like many toxic plants, there's no 'eat me' look about the plant.

These days farmers know to remove it from grazing and, with modern milk processing, any milk that is toxic gets diluted to harmless levels by mixing with many other cows' milk.

There could be an increase in deaths if more people decide to 'go back to nature' and start drinking raw milk from their own cow.

This is a noxious weed that crops up in my garden now and again. I have to stay diligent in keeping it out of my garden. Definitely not good if you have a small pet that likes to nibble on random things in the garden. I wouldn't recommend handling this plant at all. Use heavy garden gloves or a shovel to eradicate it. As indicated in previous discussion, it can easily be mistaken as there are several 'look-a-likes'. Milksickness, though not as common, is still an issue and it's a horrible, absolute treacherous way to go, including: tremors, severe abdominal pain, cramping, continuous vomiting, dizziness, coma & finally death. No 2nd chances.

I found all this an interesting read. It has a pretty flower. Good thing we don't have it in the U K.

It is important to have a rational fear of poisonous plants but, equally, there's no need to panic.

Though I haven't looked in recent years, when I used to study the American Poison Centres' annual reports in detail, there were rarely more than ten deaths directly attributable to growing plants (as opposed to extracts like tobacco) and I don't recall ever seeing one due to milkweed.

I posted this in reference to a previous discussion about white snakeroot, to distinguish it from other plants.