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And when it's mixed with benzoic acid? (the 'salting' process mentionned)

From Bing:

Is benzoic acid a carcinogen?

It is considered safe only at less than a .1% concentration in a finished product (Source) When benzoic acid and vitamin C (ascorbic acid) or citric acid are present, under the right conditions of heat and light, these two ingredients can combine to create the known carcinogen, benzene.

My impression is that nicotine is "essentially" harmless but caffeine is actually harmless, maybe even good for you (kind of like a glass of red wine a week).

Try taking a whole bunch of caffeine pills, definitely not harmless.

The dose makes the poison and tablets are about the only way you could overdose on caffeine. Your stomach will be telling you to stop long before you get to 40 cups, which isn’t enough for a healthy adult to overdose on.

They said the same thing about cigarettes decades ago, proving it with “scientific” studies.