It is actual cyanide, which is why it is effective against cancer. You certainly shouldn't eat a "handful" of apricot kernels without ever eating them before - in the regions where they are eaten as food the people introduce them slowly to children so that their tolerance is built up over time. Also found in bitter almonds, which all of us have eaten at some point because anything "almond" flavored comes from bitter almonds, not the sweet ones, like almond extract, which is heavily used in baking.
It's also known that poisons, such as anthrax, can safely be ingested by those who micro-dose until their tolerance is higher than normal and the effects of the poison are of no consequence to them. And, no, I'm not advocating that anyone do this, I'm simply pointing out that it can be done, and is why some people can eat them and not get sick.
Before our produce was heavily genetically modified, amygdlin was also pretty prevalent in berries - wild blackberries, blueberries, raspberries - and as a hunter-gatherer species, we clearly have been eating them for as long as we've been here. It is also found in smaller ratios in raw cashews, macadamia nuts, walnuts, flax, and pecans.
Laetrile, when used as a cancer treatment, is infused via IV, bypassing the GI tract, where the poisoning occurs. It is a very similar concept to the cancer treatment of IPT (or Insulin Potentiation Therapy). With IPT the cancer cells are tricked into up-taking the "targeted chemo" by the insulin potentiate. So, in simpler terms, the cancer cells think they're getting a sugary snack because of the rush of insulin, so they open up all their little receptors, and the following chemo is sucked right in, avoiding normal healthy tissue - the cancer is killed and the patient has much fewer side effects than non-targeted chemo patients.
With IV Laetrile treatment, the cancer cells draw in the amygdalin because half of it's chemical makeup is glucose (cancer's sugary snack). The cancer releases beta-glucosidase to release the glucose for consumption, but in doing so it hydrolyzes the cyanide components, making them active. So, the cancer cell gets killed, and the patient has less side effects than a person taking normal chemo.