Medication administration is a defining element of paramedic clinical practice. Paramedics use the science of pharmacology in a variety of ways, such as when treating patients who already receive medications on an intermittent or long-term basis. Prehospital providers will encounter patients who are experiencing adverse effects of medications taken at home, so it is crucial to obtain a medication history during patient assessment. Paramedics must also understand pharmacology when administering medications to treat patient symptoms during an EMS response or while treating a patient who has been exposed to a potentially toxic chemical, drug, or medication.