These drugs must be specific and only kill a certain kind of cell, or else they would hurt the human cells too, and that would make them dangerous. But antibiotics must be used wisely. Because bacteria are living organisms, they are always changing in an effort to resist the drugs that can kill them. When antibiotics are used incorrectly, bacteria can adapt and become resistant. Antibiotics are then no longer useful in fighting them.
Antibiotic resistance is now a major public health issue.
Some strains of TB have become resistant to so many drugs now that, we are running out of new drugs with which to treat them. Once bacteria have become resistant to a certain antibiotic, it will no longer work to kill them. If you keep treating these resistant bacteria with the same drugs after they become resistant, they will just keep on growing. But, if you use another drug to which they are not resistant, then they can be killed. This is why tuberculosis is treated with more than one type of drug at a time.