Bleach can kill some drain flies on contact but it is not an effective long-term solution. Bleach slides past the organic biofilm inside pipes where larvae breed without removing it. For lasting results use enzyme-based drain cleaners that digest the organic buildup providing a permanent fix. Combine with mechanical cleaning using a drain brush. Pouring bleach regularly can also damage pipes and harm the environment. Focus on eliminating the breeding habitat rather than just killing adult flies.