There are several routes of administration of pharmacological agents including intravenous, oral, rectal, inhalation, topical, transdermal, subcutaneous, intramuscular and sublingual. All forms of treatment except intravenous require absorption, and this process is associated with the chemical nature of the drug and its size. The size is generally limited because absorption and distribution are partially governed by the physical size of the particle that must diffuse across membranes or move within a fluid. The chemical nature of the drug also has a great deal to do with its ability to pass through biological membranes . However, fat soluble molecules may freely pass through membranes by diffusion driven by a concentration gradient.
Most drugs are weak acids or bases at physiologic pH .
Therefore, the ionization state of a drug is also important in its absorption and elimination from the body. For instance, phenobarbital is removed much more rapidly from the body if the urine is alkaline, and in cases of drug overdose application of this fact may be used to increase drug elimination. The oral route is most commonly used because of convenience and efficacy of absorption. After absorption from the GI tract the blood carries the drug directly to the liver by way of the portal circulation before it passes to the rest of the body. As a detoxifying organ, the liver destroys a portion of some drugs before they have a chance to have an effect.