I know that nonstick can release chemicals / gasses / toxins after being overheated but is it safe to use again once cooled? I have a frying pan I like a lot but it got overheated a few times - there are no signs of damage although the pan is very so slightly warped (not visible unless you put a ruler over the back)
Based on a search of sources covering health risks around Teflon (PTFE) cookware, nobody has found any evidence that cooking with previously overheated cookware is dangerous to your health. Breathing the fumes during overheating is bad for you, and cookware that has been overheated may cease to be nonstick, even though it looks OK:
Teflon™ nonstick coatings will not begin to deteriorate in appearance or performance until the temperature of the cookware reaches about 500°F (260°C). The coating will not show significant decomposition unless temperatures exceed about 660°F (349°C).
... in other words, the nonstick starts to fail before there's visible damage.