Pharmacological drug-drug interactions
Pharmacological interactions: These interactions can be pharmacokinetic or pharmacodynamic.
Pharmacokinetics: This describes what the body does to the drug. It includes:
- Absorption
- Distribution
- Metabolism
- Excretion
- Drug elimination
Pharmacodynamics: This describes what the drug does to the body. It includes:
- Drug actions on receptors and the resulting effects
- Dose-response relationships (dose-response curves)
Drugs can affect the pharmacodynamics of other drugs by directly changing each other’s effects. They can also affect the pharmacokinetics of other drugs by changing how the other drug is absorbed, distributed, metabolized, or eliminated. For example, drugs may:
- Form complexes with other drugs
- Activate metabolic enzymes (e.g., Cytochromes P450)
- Compete for transporters
Types of pharmacodynamic drug-drug interactions
Additive: The combined effect of two or more drugs is equal to the sum of their individual effects (1+1=2).
Synergistic: The combined effect of two or more drugs is greater than the sum of their individual effects (1+1 >2).
Potentiation is a type of synergism where the potentiator drug has no effect on its own, but it enhances the effect of another drug. For example, combining caffeine with an opioid analgesic can reduce the required dose of the analgesic.
Antagonistic: The combined effect of two or more drugs is less than the sum of their individual effects (1+1<2) because their actions oppose each other.
Examples of additive drug interactions
| Drug combination | Effect |
|---|---|
| Quinolones + macrolides | Prolongation of QTc, arrhythmias |
| ACE inhibitors + spironolactone | Hyperkalemia |
| SSRIs + triptans | Serotonin syndrome |
| NSAIDs + glucocorticoids | Gastrointestinal bleeding |
Not all drug interactions are harmful; some are used intentionally. For example, combining medications like paracetamol with aspirin is used in combination therapies to provide increased pain relief in migraine headaches.
Examples of synergistic drug interactions
| Drug combination | Effect |
|---|---|
| Gentamicin + ampicillin | Increased bactericidal effect |
| Acetaminophen + aspirin | Increased pain control |
| Sildenafil + nitroglycerine | Severe hypotension |
When two or more drugs are antagonistic, the overall result is a reduced effect compared with what you’d expect if their effects simply added together. For example, a patient taking antihypertensive medication may have reduced blood pressure control if they also take certain herbal supplements.
Antagonistic interactions can also be used therapeutically. In drug poisoning, an antidote may work by blocking the drug’s receptor and reversing its effects.
Examples of antagonistic drug interactions
| Drug combinations |
|---|
| ACE inhibitors + NSAIDs |
| Levodopa + neuroleptics |
| Naloxone reverses the effect of narcotics |
| Flumazenil reverses the effect of benzodiazepines |