Oral ingestion is the most common route of administration of pharmaceuticals. Passing through the esophagus to the stomach, the contents of the capsule or tablet are absorbed by the GI tract. The absorbed pharmaceutical is then passed through the liver and kidneys.
The rate of dissolution is a key target for controlling the duration of a drug's effect, and as such, several dosage forms that contain the same active ingredient may be available, differing only in the rate of dissolution. If a drug is supplied in a form that is not readily dissolved, it may be released gradually and act for longer. Having a longer duration of action may improve compliance since the medication will not have to be taken as often.  Additionally, slow-release dosage forms may maintain concentrations within an acceptable therapeutic range over a longer period, whereas quick-release dosage forms may have sharper peaks and troughs in serum concentration.
The rate of dissolution is described by the Noyes–Whitney equation as shown below:
Where:
* is the rate of dissolution.
*A is the surface area of the solid.
*C is the concentration of the solid in the bulk dissolution medium.
* is the concentration of the solid in the diffusion layer surrounding the solid.
*D is the diffusion coefficient.
*L is the diffusion layer thickness.
As can be inferred from the Noyes–Whitney equation, the rate of dissolution may be modified primarily by altering the surface area of the solid by altering the particle size (e.g., with micronization). For many drugs, reducing the particle size reduces the dose needed to achieve the same therapeutic effect. The particle size reduction increases the specific surface area and the dissolution rate and does not affect solubility.
The rate of dissolution may also be altered by choosing a suitable polymorph of a compound. Different polymorphs have different solubility and dissolution rate characteristics.  Specifically, crystalline forms dissolve slower than amorphous forms since they require more energy to leave the lattice during dissolution. The stablest crystalline polymorph has the lowest dissolution rate. Dissolution also differs between anhydrous and hydrous forms of a drug. Anhydrous forms often dissolve faster but sometimes are less soluble.
Esterification is also used to control solubility. For example, stearate and estolate esters of drugs have decreased solubility in gastric fluid. Later, esterases in the gastrointestinal tract (GIT) wall and blood hydrolyze these esters to release the parent drug.
Coatings on a tablet or pellet may act as barriers to reducing the dissolution rate. Coatings may also be used to control where dissolution takes place. For example, enteric coatings only dissolve in the basic environment of the intestines.
Drugs held in solution do not need to be dissolved before being absorbed.
Lipid-soluble drugs are absorbed more rapidly than water-soluble drugs.