First order kinetics describes a process where the rate of reaction is directly proportional to the concentration of the reactant. The rate increases linearly as the concentration increases. The concentration decreases over time but the reaction is never complete. Drug metabolism often follows first order kinetics, where a constant fraction of the drug is metabolized per unit of time. This can be expressed by the rate equation dC/dt = -kC, where C is the concentration and k is the rate constant. The half-life of a first order reaction is calculated from the rate constant. Pseudo-first order kinetics can also describe drug elimination from tissue where high concentrations of tissue components interact with drugs.

2. FIRST ORDER KINETICS

3. First order kinetics
The process that is directly proportional to the drug
concentration available at that time.
• Linear kinetic process
• Concentration dependent process
• Rate of process increase linearly with increase in
drug concentration
• Never comes to an end

4. Drug metabolism
First order kinetics
Constant fraction per unit
time is metabolized
Rate increases if drug conc.
increases

5. General expression
• dC/dt = - kC
Here ,
dC = The change in concentration
dt = Time interval
dC/dt = The velocity of the reaction
k = Rate constant
C = Concentration

6. General equation: Log C = Log C0 - kt / 2.303
Half-life : t1/2 = 0.693 / K
- Concentration independent
- Constant value
Rate constant units : Time‾ 1

7. www.chem.purdue.edu

8. This is the same curve as the one above
but the y-axis is the natural logarithm of conc.
This allows the calculation of t½.
http://schoolbag.info/chemistry/mcat_2/
mcat_2.files/image252.jpg

9. Pseudo-order reaction
• Following administration of a drug, it may be
eliminated from the body only after reacting with
tissue components.
-Tissue components present in high
concentration.
-Are not used up to any degree.
-Show pseudo-monomolecular reactions
-Drug eliminates from the body according to first
order kinetics.

10. Example:
100 mg drug orally
Absorption rate: 10% per
hour
First hour : 10 % of 100
mg(10 mg)
Second hour:10% of
remaining 90mg (9 mg )
Third hour : 10 % 0f
remaining 81 mg (8.1 mg)
and will continue in this
process. http://www.aboutkidshealth.ca//En/Asse
ts/PST_child_medicine_syringe_EN.jpg

11. References
• IUPAC, Compendium of Chemical Terminology, 2nd
ed. (the "Gold Book") (1997). Online corrected
version: (2006)
• http://www.pharmainfo.net
• http://www.chemteam.info
• http://chem.libretexts.org