1. Oral : Drugs are taken through the mouth and absorbed via the gastrointestinal tract

2. Rectal : Administered through the rectum , useful when oral administration is not feasible

3. Topical : Applied to the skin or mucous membranes , such as creams or eye drops

4. Injection : Includes intravenous ( IV ) , intramuscular ( IM ) , and subcutaneous ( SC ) methods for direct delivery into blood or tissue

5. Respiratory : Inhaled drugs ( e.g., nasal sprays, inhalers ) absorbed through the respiratory tract

O - R - T - I -R

1. Intervascular = blood plasma within blood vessels

   • 3 Liters = 4%

2. Interstitial = fluid surrounding the cells in tissues

   • 9 Liters = 13%

3. Intracellular = fluid inside cells ( cytosol )

   • 28 Liters = 41%

1. Absorption : Passage through membranes , impacted by drug solubility and ionization

2. First-Pass Metabolism : Degradation in the gut wall or liver before reaching systemic circulation

3. Chemical Stability : Destruction of the drug in the gastrointestinal tract ( e.g., stomach acid )

1. Phase I :

   • Convert parent compound into a more polar / hydrophilic metabolite

   • Involves oxidation, reduction, or hydrolysis to add or unmask functional groups ( e.g., -OH , -NH2 ) , often preparing the drug for Phase II

2. Phase II : Conjugation reactions with endogenous substrates to further increase water solubility for excretion.

   • conjugation with glucoronide , sulfate , acetate , amino acid

   • try to make them even more polar , to get them out of the body and into the urine

   • Phase II is the true “detoxification” step in the metabolism process

1. Renal Excretion : Through urine , involving filtration , secretion , and reabsorption in the kidneys

2. Biliary Excretion : Through bile , often leading to fecal elimination

3. Respiratory Excretion : Drugs exhaled through the lungs , especially volatile compounds