The medications gradually break down and become ineffective. Make fresh preparation each morning. Avoid higher doses when medicating. Just because a little bit is good doesn't mean that more is better. The recommended dosage should be adhered to.
Some drugs are very toxic at higher levels.
Remember, when treating via the drinking water that the dose should be adjusted depending on the volume of water consumed. The recommended doses are for mild temperatures. Use a little less in hot conditions, and up to twice the dosage in very cold weather when the birds drink much less. Avoid using drugs at lower than recommended doses and for less than the recommended time. This practice allows the organisms to become tolerant or resistant to the medications by being exposed to them for insufficient time or at levels too low to elimimate them.