Exemplifying few pharmacodynamic interactions.

1. PHARMACODYNAMIC
INTERACTIONS
MASH’HOOD MAHMOOD KHAN SHAHID

2. DRUG INTERACTIONS
• Drug interaction is said to occur if effect of one drug is altered by co-administration of another drug,
herbal medicine, food or other environmental chemicals.
• These could be clinically relevant or irrelevant.
• These are classified as;
• Pharmaceutical
• Pharmacokinetic
• Pharmacodynamic
MAJOR CAUSES:
1. POLYPHARMACY
2. MULTIPLE PRESCRIBERS
3. COMORBIDITIES
4. ERROR IN TAKING DRUG
5. NARROW THERAPEUTIC INDEX
DRUGS

3. PAHRMACODYNAMIC INTERACTIONS
• Pharmacodynamic interactions are those in which effects of one drug are altered by the other drug at its
site of action.
• Mechanistically , these interactions are either DIRECT or INDIRECT interactions.
• These interactions could be;
• Loss of therapeutic effect
• Toxicity
• Unexpected increase in pharmacological activity.

4. MECHANISMS
Direct interactions
• Drugs with similar or opposing pharmacological
actions are employed.
• These are categorized as;
• Additive/Synergism
• Potentiation
• Antagonism
Indirect interactions
• Both of the drugs have unrelated effects,
however, ultimate outcome of combination is the
incidence of interaction.
• These interactions are of major, moderate or
minor status based upon adversity.

5. EXAMPLES

6. EXAMPLES
ADENOSINE THEOPHYLLINE
THEOPHYLLINES MAY
ANTAGONIZE THE
CARDIOVASCULAR EFFECTS
OF ADENOSINE.

7. EXAMPLES
NSAID WARFARIN
INCREASED RISK OF
BLEEDING.

8. EXAMPLES
VERAPAMIL
BETA- ADRENERGIC
ANTAGONISTS
BRADYCARDIA & ASYSTOL

9. EXAMPLES
CLOZAPINE CO-TRIMOXAZOL
INCREASED RISK OF BONE
MARROW SUPPRESSION.

10. EXAMPLES
5-FLUOROURACIL FOLINIC ACID
EFFICACIOUS INHIBITION OF
THYMIDYLATE SYNTHETASE;
THUS MORE INHIBIOTORY
AND CYTOTOXIC TO
CANCEROUS CELLS.

11. EXAMPLES
AMINOGLYCOSIDE FUROSEMIDESYNERGISTIC OTOTOXICITY

12. EXAMPLES
WARFARIN
ASPIRIN/
FIBRATES/
THYROID HORMONE
INCREASED ANTICOAGULANT
EFFECT OF WARFARIN.

13. EXAMPLES
ACE INHIBITOR SALICYLATES
EFFECT OF ACE-INHIBITOR IS
REDUCED BY;
INHIBITION OF
PROSTAGLANDIN SYNTHESIS

14. EXAMPLES
DIGOXIN
AMIODARONE/
CLARITHROMYCIN/
POTASSIUM/
QUINIDINE
INCREASED INCIDENCE OF
DIGOXING-TOXICITY

15. EXAMPLES
K-SPARING DIURETICS
POTASSIUM
SUPPLEMENTS
HYPERKALEMIA;
IF SERIOUS MAY LEAD TO
CARDIAC FAILURE AND EVEN
DEATH

16. EXAMPLES
ACETAMINOPHEN ALCOHOL
INCREASED RISK OF
ACETAMINOPHEN INDUCED
LIVER DAMAGE

17. WARFARIN
GARLIC/
GINGER
INCREASED RISK OF
BLEEDING
ANTI-CONVULSANTS WORMWOOD
DECREASE IN SEIZURE
THRESHOLD
DIGOXIN HAWTHORNE
INCREASED DIGOXIN
TOXICIITY

18. INTERVENTIONS
There are many possible interventions to avoid or minimize the risk of drug interactions;
• Thorough review of patient history and identify patient risk factors.
• Be knowledgeful about the actions of drugs being used.
• Switch one of the potential interacting drugs.
• Allow a gap between administration of interacting drugs.
• Alter the dose of one interacting drug.
• Advise patient to seek guidance if they are going to plan a change in lifestyle or medications (as herbal or
supplements).
• Education of patient and monitor history.

19. REFERENCES
• DRUG INTERACTION FACTS 2009
• CLINICAL PHARMACY AND THERAPEUTICS (BY ROGER WALKER)
• BASIC AND CLINICAL PHARMACOLOGY 13TH EDITION (BERTRAM G. KATZUNG)

20. THANKS