It says so in multiple dictionaries.
Here are some dictionary definitions of 'poison'
http://www.thefreedictionary.com/poison
poi·son
(poi′zən)
n.
1. A substance that causes injury, illness, or death, especially by chemical means.
2. Something destructive or fatal.
poison
(ˈpɔɪzən)
n
1. (Medicine) any substance that can impair function, cause structural damage,or otherwise injure the body.
poi•son
(ˈpɔɪ zən)
n.
1. a substance that has an inherent tendency to destroy life or impair health.
(if anyone doubts that sugar can be used to destroy life or impair health, then they need to know that sugar is one of the best preservatives ever known. It is the only preservative used in jam making, and has been used for centuries. Obviously, the amount/proportion of sugar used is important, but then anyone who carb counts knows that already).
and here is a dictionary definition of the word 'toxin' from the same website
tox·in
(tŏk′sĭn)
n.
1. A poisonous substance, especially a protein, that is produced by living cells ororganisms and is capable of causing disease when introduced into the bodytissues but is often also capable of inducing neutralizing antibodies or antitoxins.
2. A poisonous or harmful nonbiological substance, such as a pollutant.
I used to object when people claimed that 'carbs are poison!' I used to flinch and think it was melodramatic and inaccurate.
However, having done this dictionary search, and knowing that we are facing a world epidemic of diabetes (and it isn't just 0.5% of the population who are negatively affected by ingesting modern carb quantities) my views have changed. There are now a quarter to a third of all American adults classed as being pre-diabetic or diabetic, and the 2016 annual figures of diagnosed diabetics in the UK was over 3million
https://www.diabetes.org.uk/Professionals/Position-statements-reports/Statistics/
The more people who realise that the modern high carb lifestyle is damaging to their health (the very definition of poison) the better.