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((more sources|date=July 2017))
'''Liberation''' is the first step in the process by which medication enters the body and liberates the  [[Medication|active ingredient]] that has been administered. The [[pharmaceutical drug]] must separate from the  [[Drug carrier|vehicle]] or the [[excipient]] that it was mixed with during  [[Pharmaceutical manufacturing|manufacture]]. Some authors split the process of liberation into three steps: disintegration, disaggregation and dissolution. A limiting factor in the adsorption of pharmaceutical drugs is the degree to which they are ionized, as cell membranes are relatively impermeable to ionized molecules. 

The characteristics of a medication's excipient play a fundamental role in creating a suitable environment for the correct absorption of a drug. This can mean that the same dose of a drug in different forms can have different [[bioequivalence]], as they yield different plasma concentrations and therefore have different therapeutic effects. [[Dosage form]]s with [[modified-release dosage (medicine)|modified release]] (such as delayed or extended release) allow this difference to be usefully applied.

== Dissolution ==
In a typical situation, a pill taken orally will pass through the [[oesophagus]] and into the [[stomach]]. As the stomach has an aqueous environment, it is the first place where the pill can dissolve. The rate of dissolution is a key element in controlling the duration of a drug's effect. For this reason, different forms of the same medication can have the same active ingredients but different dissolution rates. If a drug is administered in a form that is not rapidly dissolved, the drug will be absorbed more gradually over time and its action will have a longer duration. A consequence of this is that patients will comply more closely to a prescribed course of treatment, if the medication does not have to be taken as frequently. In addition, a slow release system will maintain drug concentrations within a therapeutically acceptable range for longer than quicker releasing delivery systems as these result in more pronounced peaks in plasma concentration.

The dissolution rate is described by the [[Noyes–Whitney equation]]:

<div class="center"><math>\frac{dW}{dt} = \frac{DA(C_{s}-C)}{L}</math></div>

Where:
* <math>\frac{dW}{dt}</math> is the dissolution rate.
* A is the solid's [[surface area]].
* C is the concentration of the solid in the bulk dissolution medium.
* <math>C_{s}</math> is the concentration of the solid in the [[diffusion]] layer surrounding the solid.
* D is the diffusion [[coefficient]].
* L is the thickness of the [[diffusion layer]].

As the solution is already in a dissolved state, it does not have to go through a dissolution stage before absorption begins.

== Ionization ==
((Main | Ionization))
Cell membranes present a greater barrier to the movement of ionized molecules than non-ionized liposoluble substances. This is particularly important for substances that are weakly  [[Amphoterism|amphoteric]]. The stomach's acidic [[pH]] and the subsequent alkalization in the [[intestine]] modifies the degree of ionization of acids and weak bases depending on a substance's [[pKa]].<ref name=anestesia>Simonetta Baroncini, Antonio Villani, Gianpaolo Serafini ''[https://books.google.com/books?id=PlWHnc1bU7IC Anestesia neonatal y pediátrica]'' (in Spanish). Published by [[Elsevier]] España, 2006; page 19. ((ISBN|84-458-1569-5))</ref> The pKa is the pH at which a substance is present at an equilibrium between ionized and non-ionized molecules. The [[Henderson–Hasselbalch equation]] is used to calculate pKa.

== See also ==
((div col))
* [[Absorption (pharmacokinetics)]]
* [[ADME]]
* [[Bioequivalence]]
* [[Distribution (pharmacology)]]
* [[Elimination (pharmacology)]]
* [[Generic drugs]]
* [[Metabolism]]
* [[Pharmacodynamics]]
* [[Pharmacokinetics]]
* [[Pharmacy]]
((div col end))

== References ==
((reflist|30em))

 ((Pharmacology))

[[Category:Pharmacokinetics]]

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