Drug-resistant TB has become a very serious problem in recent years in certain populations. For example, TB resistant to the drug isoniazid (INH) is seen among patients from Southeast Asia. The reason for this INH resistance is not quite clear. However, the presence of INH-like substances in the cough syrups in that part of the world may play a role in causing the INH resistance.
An even more serious problem is the multidrug resistant TB that has been seen in prison populations. Poor compliance (failure to take the prescribed drugs) by the inmates is thought to be the main reason for this multidrug resistance.