Endorphins
HCO3 is the medical and chemical abbreviation meaning bicarbonate.
Chemotherapy is the medical term meaning chemical treatment.
Toxicology
Chemotherapy is the medical term meaning chemical treatment.
Pica is the medical term meaning persistent eating of non-nutritive substances such as clay, starch, or cigarette butts.
catabolism
Hydrophilia is a chemical term meaning attraction to water.
If by natural you mean found in nature, most analgesics are based on chemical processes or are made by refining natural substances into those which are useful. For example, acetylsalicylic acid, or Aspirin as it is commonly known, was first isolated from willow bark. Morphine many similar analgesics are derived from Opium, which is obtained from poppy seeds. If you mean holistic or alternative medicine pain killers, there are none of which I am aware which have been proven in studies to be as effective as those used in medical practice.
The medical term you are looking for is "nuclear medicine." This branch of medicine uses radioactive substances to diagnose and treat various diseases and conditions in the body.
The suffix meaning urine is "-uria." It is commonly used in medical terminology to describe conditions related to urine or the presence of substances in the urine.
Aspiration means withdrawal of fluid by Vacuum or suction
any of a large group of chemical substances, as penicillin or streptomycin, produced by various microorganisms and fungi, having the capacity in dilute solutions to inhibit the growth of or to destroy bacteria and other microorganisms, used chiefly in the treatment of infectious diseases.