Drug Interactions
General activity of a drug is affected when its is administered with another drug or food, herbal medicine etc, this is termed as Drug Interaction.
Many drug interactions are harmless and the percentage of harmful interactions is very less also the severity of an interaction varies from one patient to another.
Mainly there are 2 types of interactions :
Synregism
Interaction between the drugs can cause an increase in the effect of one or both drug.
Antagonism
On interaction when a drug blocks the effect of another drug is known as antagonism.
Nature of interactions may be Pharmacodynamic or Pharmacokinetic.
Pharmacodynamic Interaction
Interactions are happening between drugs of same class or drugs with similar pharmacological effects. Drugs of same nature show Competition at the receptor sites or at the physiological site and this is the main reason for Pharmacodynamic interaction.
Pharmacokinetic Interaction
Interactions of one drug alters the absorption, distribution, metabolism, or excretion of another drug, thus it alters (increases or decreases) the availability of to produce its pharmacological effects.
- Interaction due to altered absorption : Rate of absorption or the total amount absorbed can both be altered by drug interactions.
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Interaction due to altered distribution : To a variable extent most drugs are loosely bound to plasma proteins. Protein-binding sites are non-specific and one drug can displace another thereby increasing its proportion free to diffuse from plasma to its site of action. Even so displacement rarely produces more than transient potentiation because this increased concentration of free drug results in an increased rate of elimination.
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Interaction due to altered Metabolism : Drugs are mainly metabolised in the liver (by hepatic microsomal system). By blocking the hepatic microsomal enzyme system, one drug can gradually increase the rate of metabolism of another, resulting in lower plasma concentrations and a reduced effect. Also one drug inhibits the metabolism of another drug thus higher plasma concentrations are produced, rapidly resulting in an risk of toxicity.
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Interactions due to altered execration : Drugs are eliminated through the kidney through glomerular filtration and active tubular secretion. Some time competition occurs between those which have the same excretory mechanism ( i.e. active transport mechanisms in the proximal tubule) here one drug delays the excretion of another drug, toxicity of the delayed drug increases.
Examples of Some important Interactions
- A severe and life-threatening toxic reaction may occur when a patient who is taking aspirin regularly is given with acetazolamide ( carbonic unhydrase inhibitor). And this is because acetazolamide increases the lethal toxicity of aspirin by altering the plasma PH, because of that aspirin exists in blood as lipid - soluble form (i.e unionized form) so that it can cross blood brain barrier and enters into CNS and other tissues easily, leading to salicylate toxicity.
- Chances for severe liver damage are high when a person who is in treatment with paracetamol is taking alcohol everyday. Paracetamol can cause liver damage, drinking three or more alcoholic drink every day can cause severe liver damage.
- ACE inhibitors like Enlapril, Captopril, Lisinopril etc may cause Hyperkalemia in regularly using patients and severe hyperkalemia is a medical emergency situation so avoid taking fruits and other food rich in potassium ( like banana, tender coconut, orange and green leafy vegetables) along with this ACE inhibitors.