Drugs that activate receptors in the brain are termed agonists. Agonists bind to receptors and turn them on. They produce an effect in the organism. Full mu opioid agonists activate mu receptors. Increasing doses of full agonists produce increasing effects until a maximum effect is reached or the receptor is fully activated. Opioids with the greatest abuse potential are full agonists (e.g., morphine, heroin, methadone, oxycodone, hydromorphone).