Drug Interactions

      Definition: Drug Interaction refers to the modification of response of one drug by another when they are administered simultaneously.  

OR : Modification of drug response by other substance (food, drug, herbal product) which is simultaneously taken or administered to the patient.

Modification of response is usually quantitive  i.e increases or decreases but sometime it is so that it appears qualitative as abnormal different type of response.

Mechanism of Drug Interaction: Three Kind of Mechanisms is Involved Here:

1)    Pharmaceutical

2)    Pharmacodynamics

3)    Pharmacokinetics

Pharmaceutical Interaction: appear when two drug are chemically and physically unstable with each other and mixed together. They cause sort of interaction chemically (Chemical Reaction i.e neutralization, Hydrolysis etc) or physically (Precipitation, crystallization etc).  

Pharmacodynamics Interaction: Occur when the drug influence the response of each other at receptor level. The two drugs may or may not act on the on the same receptor to produce such effects. Theoretically drugs acting on the same receptors are usually additive. Drugs acting on different receptors may be agonistic, antagonistic or synergistic etc. Pharmacodynamic interactions may be additive, synergistic or antagonistic effect. 

Pharmacokinetic Interactions : may occur when one drug affect the pharmacokinetic processes of the other. Pharmacokinetic processes involves absorption, distribution and elimination (excretion + metabolism).  Sometime drug interactions may be due to one mechanism or combination of these.

Drug Interaction Effects:

They effect of drug interaction may be Additive effect (increase in response), Antagonistic effect (decrease in response) and No effect.

Increase in Effect: this may also be called additive effect.  Additive effect may be useful or harmful.

·       Beneficial Additive Effects : Beneficial additive effects are usually used for the purpose of therapeutic purposes. Just like different anti-hypertensives are give to reduce blood pressure effectively. Same is the case of antibiotic combination and Antidiabetic drug. These combination give batter therapeutic response as compare to single drug administration.

·       Harmful Additive effects: In this case the additive effect dose not goes in favor of patient and give harms to the body. Just like Benzodiazepines and Anti-Histamine may lead to prolongation of sedative and hypnotic effect or may case other effect just like respiratory depression etc. or combination of beta blockers and calcium channel blockers (verapamil) may lead to heart block due to additive effect on Nodal blockage.

Antagonistic effect (Lessened Effect): in this type of interaction the effect of one drug is decreased or antagonized by the presence of other drug. Same as Agonistic effects the antagonistic effects may be beneficial or harmful.

·       Beneficial Antagonistic: Some time antagonism is used for beneficial purposes. Just like in case of the toxicities the effect is antagonized by administering anti-dotes which usually work antagonistically to the original drug. For Example in case of Benzodiazepine toxicity flumazenil is used. Flumazenil works antagonistically to the Benzodiazepine.

·       Harmful Antagonistic effects: Harmful antagonistic effects are usually not accepted. In this case one drug decreases or completely block the required therapeutic effect of the other drugs. Just like in case of NSAIDS (Indomethacin and ibuprofen) Reduces the effect on antihypertensive.

No-Effect: No effect is said when the effect of the drug interaction is very minor or no significant change.