The evolutionary arms race has made milkweed unpalatable to grazing animals. But of course, the monarch butterfly has adapted, and specializes in eating milkweed. The butterflies, in their turn, use the bitter end emetic chemicals to protect themselves. The caterpilars store the bitter chemicals in their parts that will become wings. They store the emetic chemicals in the parts that will be the body of the adult butterfly. If a predator tastes the wings, they will usually stop there, even before they cause so much damage that the butter can't fly. If they get past the taste of the wings, and eat the body, the reaction soon teaches them to avoid monarchs.
As you can see in this image, this is indeed a common milkweed. The flowers don't contain the active phytochemicals. They are sweet, and were used by Indians to sweeten food. They also attract all sorts of pollinators. They are a favorite of aphids, which suck up the sweet sap. Those aphids are in turn a favorite of ants and other animals. Ants actually farm aphids on milkweeds. They will carry the wingless aphids around to different flowers, or even plants, as the first sources of sweetness are depleted.