BCG Adenitis. Effects of taking tablet?

Hello Doctor, My child had got BCG lymphadenitis in left arm when he was 2 months old. Our doctor removed it surgically after puss had increased. It is getting healed now. He had also prescribed us to take 'Solonex 100mg (Isoniazid)' tablet half a day for 2 month duration. We have continuously taken it for two months. He is 4.5 months old. We had to move to another location and hence we consulted another doctor now. This new Doctor is saying Solonex needs to be given for 6 months. I am afraid if this dosage will be huge. I want to know if we have to take for 6 months. Are there are any side effects of taking tablet? If we visit another doctor , they might prescribe other tablets. We are little worried if we need to continue taking tablets.