The drug has lost its potence by being exposed to moisture.
这药因受潮失去了效力。
《新英汉大辞典》
2
The accepted scenario was a simple case of evolutionary selection. In a bacterial population exposed to a killer drug, a few lucky individuals might have a genetic mutation that kept them alive.
In the process, the virus glows green when exposed to an ineffective drug, thus allowing rapid distinction between ineffective and potentially effective drugs against smallpox.