1) Drug interactions can occur when two drugs are taken together and one drug modifies the effects of the other. This can increase therapeutic effects in some planned cases, but can also cause toxicity or therapeutic failure in unplanned cases. 2) Mechanisms of drug interactions include changes in absorption, distribution, metabolism and excretion of one or both drugs. They can occur through pharmacological or physiological pathways such as enzymatic induction or inhibition. 3) Several case studies are presented involving patients taking multiple drugs where evaluating for potential drug interactions is important for optimizing treatment and avoiding adverse effects.

1. Drug interaction
Prepared
By
Cl.ph: Joseph Talaat William
Clinical Pharmacist in Assiut University Hospitals

2. Definition
O Means changes in the effects or
concentrations of one drug by another
modifying agents when administered
concomitantly.
O The modifying agents may be:
1- Drugs.
2- Foods.
3- Smoking ……etc.

3. Clinical significance of drug
interaction
1- Planned (Desired):
To increase therapeutic effects e.g. Thiazides
and Spironolactone.
2- Unplanned (Undesired or Hazards):
If one drug increases or decreasing the
concentration of the other causing toxicity or
therapeutic failure respectively. e.g., digoxin
and spironolactone.

4. Results of drug interactions
1- Antagonism:
One drug decrease or neglectable the effect of
second drug .
(20+20=10) or (20-20=0)
2- Simple addition or summation:
Each act separately without affecting the
effect of the second drug .e.g., propranolol +
thiazide diuretics.
(20+20=40)

5. Cont.…
3- Potentiation:
One drug has no therapeutic effect if used
alone but can increase the effect of second
drug.
(0+20=40)
e.g., penicillin+ b-lactamase inhibitor

6. Cont.…
4- Synergism:
Each drug increase the effect of the second
drug.
(10+10=40)
e.g., sulfonamides + trimethoprim.

7. Mechanisms or Types
of
Drug-Drug interaction
In vitro In vivo

8. In vitro
Chemical drug
interaction
Physical drug
interaction

9. In vivo
Pharmacological
drug interaction
Physiological
drug interaction

10. Pharmacological
drug interaction
Pharmacokinetics
drug interaction
(What body does to
the drug?)
Pharmacodynamics
drug interaction
(What drug does to
the body?)

11. Pharmacokinetics
Drug interaction
Interaction affecting
absorption
Interaction affecting
distribution
(Affect Plasma
protein binding)
Interaction affecting
biotransformation
Interaction affecting
excretion

12. Pharmacokinetics
drug interaction
By changing in PH
By forming
complexes
By changing the
motility of GIT
Interaction
affecting
absorption

13. Pharmacokinetics
drug interaction
Enzyme
Inducer
Enzyme
Inhibitor
Drug affecting
biotransformation
(Metabolism)

16. Pharmacodynamics
drug interaction
Agonists Antagonists

17. Case studies

20. Case (1)
*Female patient complaining of nausea and
symptoms of a urinary tract infection in early
pregnancy. The organism is reported as being
resistant to ampicillin but sensitive to:
1-Nitrofurantoin.
2-Gentamycin.
3-Tetracycline.
4-Trimethoprim.
5-Cefotaxime or Ceftriaxone.
O Which one of these ABs would be
referred to this patient?

21. Case (2)
*Hospitalized male patient has a symptoms
of a urinary tract infection and take Warfarin
as oral anticoagulant his laboratory test show:
1- Low Na , K & Ca serum level.
2- increase in liver enzyme.
3- increase in bilirubin level.
Doctor give the patient Ringer solution to
correct his electrolytes disturbance.

22. Cont.…
*The organism which causes UTI is reported
sensitive to:
1- Augmentin.
2- Ceftriaxone.
3- Cefotaxime.
4- Cefoprazone.
O Which one of these ABs would be
referred to this patient?

23. Case (3)
• 36 yr. hospitalized male patient has UTI and
take Ciprofloxacin 500 mg as antibiotic
every 12 hr. , and take Ferrotron Cap
once daily because he has anemia .
• After one week, the patient still has UTI
and DR refer the patient to you to
chose the best AB.

24. Case (4)
• 40 yr. female patient has fungal infection
and take ketoconazole 100 mg twice daily
before meal and has gastric pain so take
Antodine 20mg twice daily.
• After one week, the patient still has
fungal infection and DR refer the
patient to you to chose the best AF.

25. Case (5)
• Male patient has a kidney stones after clinical investigation
show the patient has Ca. oxalate stone , pus and some RBC
in the urine.
• DR give him this medications:
O Rx /
Tavanic 500 mg tab .
Every 24 hr. before meal.
Xenomag eff.
3 times daily before meal.
Urinex cap.
3 times daily before meal.
Peroximol eff.
3 times daily before meal.

26. Cont.…
O After two week, the patient still has
pain and lab investigation show no
improve in the case and DR refer the
patient to you to show any drug
interaction in the RX.

27. Case(6)
*A 65 yr. black ,obese hospitalized male
patient has hypertension take Concor 5 mg .
O After 4 days the B.P (150/100) ,DR change
Concor 5 mg to Enapril 10 mg at once.
O After one week the B.P (170/100) .

28. Case (7)
• 25 yr. old female patient has chest infection
and culture show organism sensitive to
Erythromycin 500mg and the patient has
hypertension and take Carvid 25mg and
Aldactone 100mg.
• After 5 days the patient fell headache and
menstrual irregularities.