An investigation was carried out to explore the possible reason for the physical instability of a marketed strip packaged anti-TB fixed dose combination (FDC) tablet containing 300 mg of isoniazid (H) and 800 mg of ethambutol hydrochloride (E). The instability was in the form of distribution of white powder inside the strip pockets. High-performance liquid chromatography (HPLC) and liquid chromatography-mass spectrometry (LC-MS-MS) studies confirmed that both H and E were present in the powder. The same was also confirmed through Fourier-transform infrared (FTIR) spectroscopy, which also indicated absence of interaction between the two drugs. No sublimation of the drugs was observed up to 110 degrees C, indicating that the observed instability was not due to this reason. Subsequently, attention was paid to the possibility of moisture gain by the tablets through defective packaging (which was established) due to hygroscopicity of E. To understand the phenomenon further, pure drugs and their mixtures were stored under accelerated conditions of temperature and humidity [40 degrees C/75% relative humidity (RH)] and both increase in weight and physical changes were recorded periodically. The mixtures gained moisture at a higher rate than pure E and those with higher content of E became liquid, which on withdrawal from the chambers, became crystallized. The drug mixture containing H:E at a ratio of 30:70 w/w, which was similar to the ratio of the drugs in the tablets (27:73 w/w), crystallized fastest, indicating formation of a rapid crystallizing saturated system at this ratio of the drugs. It is postulated that the problem of instability arises because of the formation of a saturated layer of drugs upon moisture gain through the defective packaging material and drying of this layer with time. The study suggests that barrier packaging free from defects and alternatively (or in combination) film coating of the tablets with water-resistant polymers are essential for this formulation.