Refers to the passage of drug molecules from the site of administration into the circulation.
The barrier is formed by tight junctions between the capillaries, which inhibit the penetrations of polar molecules into the brain neurons
Drug interactions may be caused by changes in the pharmaceutical, pharmacodynamics, or pharmacokinetic properties of the affected drug.
1- biotransformation( metabolism) of a drug to one or more metabolites, primarily in the liver
2- excretion of the parent drug or its metabolites primerly by the kidneys.
Drugs that are absorbed from the gut reach the liver via the hepatic portal vein before entering the systemic circulation.
Many drugs, are extensively converted to inactive metabolites during the 1st pass through the gut wall and liver and have low bioavailability
Drugs administer by the sublingual or rectal route undergo les first -pass metabolism and have a higher degree of bioavailability than drugs administered by the oral route.
Solutions are absorbed more rapidly than particle suspensions, so suspensions are often used to extend the duration of action of drug over many hours or days.
Most drugs are absorbed more rapidly after intramuscular than after subcutaneous administration because of the greater circulation of blood to the muscle.
The oral administration is convenient, relatively safe, and the most economical.