The pharmacokinetics of a substance can be understood by considering its (liberation), absorption, distribution, metabolism, and excretion (ADME). While intravenously administered substances are directly available in the systemic circulation for distribution, substances administered e.g. orally, subcutaneously, or via inhalation need to be absorbed first. Also, in some cases the substance might not be directly available for absorption, but needs to be liberated first, in case an extended LADME scheme is of relevance.