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Dose Response Relationship, Therapeutic Index, Combined Effects of Drugs and Factors Modifying Drug Action

This relationship can be linear or nonlinear. In general, the higher the dose of a drug, the greater the response.

Dose-response Relationship

The dosage-response link is the connection between a drug's dose and the response it generates.
  • This relationship can be linear or nonlinear.
  • In general, the higher the dose of a drug, the greater the response.
  • However, there can be diminishing returns at very high doses, where the increase in response is not proportional to the increase in dose.
  • The dose-response relationship is often represented by a curve, with the response plotted on the y-axis and the dose on the x-axis.

Factors that affect Dose-response Relationship

1. Receptor number and affinity: The number of receptors for a drug directly corresponds to the drug's reaction.
2. Receptor occupancy: The response to a drug is also directly proportional to the percentage of receptors occupied by the drug.
3. Rate of receptor desensitization: Receptors can become desensitized over time, which reduces the response to a drug.
4. Rate of receptor internalization: Receptors can be internalized after binding to a drug, which reduces the number of receptors available for subsequent interactions.
5. Second messenger systems: Second messenger systems can amplify or attenuate the response to a drug.
6. Gene expression: The response to a drug can be controlled by the word of genes involved in drug metabolism or response.

Therapeutic Index

  • The therapeutic index (TI) is a measure of the relative safety of a drug.
  • It is calculated by dividing the LD50 by the ED50.
  • The higher the TI, the safer the drug.
  • A drug with a TI of 10 has a 10-fold safety margin.
  • A higher therapeutic index indicates a more favorable safety profile, as it takes a higher dose of the drug to produce toxicity.
However, a lower therapeutic index does not necessarily mean unsafe drugs.

Aspects that may have an impact on the therapeutic index include

1. The route of administration: Drugs administered orally have a higher therapeutic index than injected drugs.
2. The dose: A higher dose of a drug will increase the risk of toxicity.
3. The duration of exposure: The longer a person is exposed to a drug, the greater the risk of toxicity.
4. The frequency of exposure: The more often a person is exposed to a drug, the greater the risk of toxicity.
5. The individual's sensitivity: Some people are more sensitive to a drug than others and may experience toxicity at lower doses.
6. The drug's toxicity: Some drugs are more toxic than others and can cause serious side effects at lower doses.
7. Interaction with other drugs: The effects of a drug may be increased or decreased by the presence of other medications.
8. The route of elimination: The faster a drug is eliminated from the body, the lower the risk of toxicity.

Combined Effects of Drugs

The effects of two or more drugs can be additive, synergistic, or antagonistic.

Additive effects

The effects of two drugs are added together.
effect of drugs M + N = effect of drug M + effect of drug N



Examples of additive effects

1. Two drugs that increase heart rate will result in a more significant increase in heart rate than either drug alone.
2. Two drugs that decrease blood pressure will result in a more significant decrease in blood pressure than either drug alone.

Synergistic effects

The combined effects of two drugs exceed the sum of their particular products.

effect of drugs M + N > effect of drug M + effect of drug N

Examples of synergistic effects

1. Two drugs that increase heart rate will result in a more significant increase in heart rate than either drug alone.
2. Two drugs that decrease blood pressure will result in a more significant decrease in blood pressure than either drug alone.

Antagonistic effects

Two drugs' combined effects are lesser than their separate results.
effect of drugs M + N < effect of drug M + effect of drug N

Examples of antagonistic effects

1. Two drugs that increase heart rate will result in a greater increase in heart rate than either drug alone.
2. Two drugs that decrease blood pressure will result in a more significant decrease in blood pressure than either drug alone.

Factors Modifying Drug Action

1. Age: The effects of a drug may be different in young people than in older people.
2. Gender: The effects of a drug may vary between men and women.
3. Pregnancy: The effects of a drug may be different in pregnant women than in non-pregnant women.
4. Disease: The effects of a drug may be different in people with diseases than in people without conditions.
5. Genetic factors: The effects of a drug may be different in people with different genetic backgrounds.
6. Ethnicity: The effects of a drug may be different in people of different ethnicities.
7. Obesity: The effects of a drug may be different in obese people than in people of average weight.
8. Renal disease: The effects of a drug may be different in people with renal disease than in people without renal disease.
9. Liver disease: The effects of a drug may be different in people with liver disease than in people without liver disease.10. Cardiovascular disease: The effects of a drug may be different in people with cardiovascular disease than in people without cardiovascular disease.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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