Final answer:
Synergistic drug interactions occur when two or more drugs used together have a greater effect than when used alone, while antagonistic drug interactions occur when the combined effect of the drugs is less than their individual effects. The drug combination being less effective than when the drugs are administered alone represents an antagonistic drug interaction.
Step-by-step explanation:
Synergistic drug interactions occur when two or more drugs are used together and their combined effect is greater than the sum of their individual effects. This means that the drug combination is more effective than when the drugs are administered alone. A classic example of synergistic drug combination is trimethoprim and sulfamethoxazole (Bactrim) which provide bactericidal effect when used together, whereas individually they only inhibit bacterial growth. On the other hand antagonistic drug interactions occur when two or more drugs are used together and their combined effect is less than the sum of their individual effects.
This means that the drug combination is less effective than when the drugs are administered alone. Antagonistic interactions can cause loss of drug activity, decreased therapeutic levels, or increased potential for toxicity. An example of antagonistic interaction is when the pH-altering effect of antacids decreases the effectiveness of certain antibacterials by affecting their absorption in the stomach. In summary the drug combination being less effective than when the drugs are administered alone is an example of an antagonistic drug interaction.