Antioxidants are substances that neutralize harmful free radicals such as oxygen radicals and non-radical oxygen containing molecules (ROS). Why are free radicals so bad? Think of them like vampires, but instead of blood, they need electrons to survive. One free radical will take an electron from a healthy molecule, which then turns it into a free radical. This is called oxidative stress. Scientists widely regard oxidative stress to be an underlying theme which exacerbate many health problems.