How Do Medicines Work?

A medicine undergoes various processes before it is able to elicit its effect. It first needs to be given through the correct route that would be most comfortable and achieves the desirable effect in a preferred time.

Common Routes of Administration:
PO (Per Oral) - This is the most common route. Medicine is placed under the tongue (sublingual) or at the cheek (buccal), chewed or swallowed.
IV (Intravenous) - Injected directly into a vein.
IM (Intramuscular) - Injected into the muscle.
SC (Subcutaneous) - Injected just beneath the skin.
Rectal - Inserting enemas or suppositories.

Absorption of Medicines:
Medicines are "absorbed" means they are "bioavailable" i.e. it is absorbed into the bloodstream. Blood is the main vehicle for medicines to reach its target site. The speed and quantity of absorption are related to the route of administration. Generally, medicines absorbed faster have an earlier onset (time taken to see the effect).

Medicines administered intravenously (IV) are 100 percent bioavailable. Intramuscular (IM) absorption is usually slow and even, but faster than subcutaneous (SC) administration. Rectal absorption is highly variable. The absorptions of oral medicines depend on their ability to pass across intestinal cell membranes, withstand stomach acid, destruction in the liver, presence of food or other medicines. Rate of absorption generally follow: liquids >suspensions >capsules >tablets (> is more than)

Medicines, which are absorbed faster, have an earlier onset (time taken to see the effect).

Distribution of Medicines:
After the drug is absorbed, it is then distributed to various organs of the body. Distribution is influenced by how well each organ is perfused (supplied by blood), organ size, binding of the drug to various components of blood and tissues, and permeability of tissue membranes. The more fat-soluble a drug is, the higher is its ability to pass across the cell membrane.

The blood-brain-barrier restricts passage of drugs from the blood into the central nervous system and cerebrospinal fluid. Protein binding (attachment of the drug to blood proteins) is an important consideration influencing drug distribution. Many drugs are bound to blood proteins such as serum albumin (the main blood protein) and are not available as active drugs.

Metabolism of Medicines:
Metabolism (breakdown) occurs via two types of reactions: phase I and phase II. The goal of metabolism is to change the active part of medications making them more water-soluble and more readily excreted by the kidney. (ie. the body is trying to get rid of the "foreign" drug) Changing the molecular structure of drugs increases their water solubility and decreases their fat solubility, which speeds up the excretion of the drug in the urine.

Phase I processes make a molecule's charge more positive or negative than the original drug. Phase II reactions involve conjugation (which means adding another compound). These reactions generally inactivate the pharmacologic activity of the drug and may make it more prone to elimination by the kidney.

Excretion of Medicines:
Excretion occurs primarily through the urine. Fecal excretion is seen with drugs that are not absorbed from the intestines or have been secreted in the bile (which is discharged into the intestines). Drugs may also be excreted in the exhaled air through the lungs, in the perspiration, or in breast milk.