Final answer:
Taking probenecid with cephalexin results in a higher concentration of cephalexin in the blood due to decreased renal excretion. This can enhance the drug's effectiveness but also raise the potential for toxicity. Antagonistic interactions occur when the combined effects of drugs are less beneficial, while synergistic interactions enhance the drugs' effects.
Step-by-step explanation:
When a patient takes cephalexin with probenecid, probenecid decreases the renal excretion of cephalexin. This interaction means that there will be a higher concentration of cephalexin in the blood for an extended period. This is because probenecid inhibits the secretion of certain drugs in the renal tubules, which slows down cephalexin's elimination from the body. As a result, cephalexin remains in the bloodstream longer, potentially increasing its effectiveness against bacterial infection but also increasing the risk of toxicity if the concentration becomes too high.
In contrast to synergistic interactions, which enhance the effect of the drugs involved, antagonistic interactions may decrease a drug's effectiveness or increase the risk of side effects. An example of a synergistic interaction is the combination of trimethoprim and sulfamethoxazole, which together can kill bacteria rather than simply inhibiting their growth. On the other hand, taking antacids with certain antibacterials can be antagonistic, resulting in decreased absorption and efficacy of the antibiotics.