Platelets, also called thrombocytes (thromb- + -cyte, blood clot cell), are a component of blood whose function (along with the coagulation factors) is to stop bleeding by clumping and clotting blood vessel injuries.
Milk of the poppy is essentially the Westeros version of morphine. Maesters make it from crushed poppy flowers, and morphine and other opiate-based drugs are made from poppy plants. Indeed, the books' descriptions strongly suggest that the poppy is intended to be the real-world opium poppy (Papaver somniferum).
Eating as little as a teaspoon of poppy seeds -- less than the amount on a poppy seed bagel -- can produce false-positive results on tests for opioid abuse, says Dwight Smith, MD, of the VA Medical Center in Black Hills, S.D.