When more than one drug are administered simultaneously:

1. They act independently of each other.

2. SYNERGISM : They produce similar actions on the same organ.

3. ANTAGONISM : They oppose each other’s action.

Derived from two Greek words( Syn= together , Ergo= work)

DEFINITION:

“Drug synergy occurs when drugs can interact in ways that enhance or magnify one or more effects, or side-effects, of those drugs.”

*****POSITIVE EFFECTS******

EXAMPLES:

1)Codeine mixed with Acetaminophen or Ibuprofen to enhance the action of codeine as a pain reliever.

2) Use of Cannabis with LSD, where the active chemicals in cannabis have been reported to enhance the hallucinatory experience of LSD.

*****NEGATIVE EFFECTS******

Negative effects of synergy are a form of contraindication. For example, a combination of depressant drugs that affect the central nervous system (CNS), such as alcohol and Valium, can cause a greater reaction than simply the sum of the individual effects of each drug if they were used separately. In this particular case, the most serious consequence of drug synergy is exaggerated respiratory depression, which can be fatal if left untreated.

1)SUMMATION:

Definition : Combined effect of drugs which are given simultaneously is equal to the sum of magnitude of effect produced by individual drugs.

Additive in nature

Examples:

General Anesthetics

2)POTENTIATION:

Combined effect of two simultaneously given drugs is greater than the algebraic sum of action of individual drugs.

Examples:

NH4Cl(Weak diuretic) potentiates the diuretic effect of Organic Mercurials.

S.A.T(Sulphamethoxazole , Antibacterials,Trimethoprim) potentiate each other and the effect is Bacteriocidal.

Definition:

Opposing action of two drugs on same biological system.

Difference between agonist and antagonist:

AGONIST: Possesses both affinity and intrinsic activity.

ANTAGONIST: Opposes the action of agonist. Possesses only affinity but no intrinsic activity.

1)Pure:

Atropine

Tubocurarine

2)Agonist-Antagonist:

Succinylcholine is an agonist itself which produces muscle fasciculations is an antagonist for acetylcholine preventing its access to the receptor so that paralysis ultimately occurs.

Chemical Antagonism PhysiologicalAntagonism

Pharmacological Antagonism

Agonist-Antagonist interaction=Agonist loses its activity

Agonist-Antagonist act on different receptors.Both have opposite actions.

Antagonist preventsagonist from acting upon its receptors.

EXAMPLES:Antacids(NaOH , Al(OH)2 ) neutralize HCl

Histamine-Adrenaline antagonism.HISTAMINE: *Vasodilation*Decreases blood pressure*BronchoconstrictionADRENALINE:*Vasoconstriction*Increases blood pressure*Bronchodilation

Two types:1) Competitive2) Non-Competitive

COMPETITIVE:1. Antagonist competes

with agonist for the same receptor site

2. Reversible phenomenon

3. Can be overcome by increasing the conc. Of Agonist.