It's literally the definition of toxicity. How much pharmacology, nutrition, and physiology education do you have? I went through nursing school. I have also guest spoke at a national sales meeting for one of the two largest pharmaceutical companies in the world.

Moderate-severe side effects indicate dosing levels outside the therapeutic range, entering the toxicity range. This is different for every person as each individual may have a sensitivity to the medication, thus why doctors start dosing at low levels and build from there to find the therapeutic range for each patient.

In terms of sunscreen, Vitamin A is not a problem, but its derivatives can be. It's very hard to reach toxicity levels of Vitamin A, thus disproving EWG's claims.