A 68-year-old, HIV negative was affected by tubercular (TB) epididymitis. His previous medical history was remarkable for a total gastrectomy with Roux-en-Y gastric bypass procedure performed five years before, due to gastric cancer. His renal and liver function was normal. His weight was 65Kg. He was treated with standard oral anti TB therapy including isoniazid 300 mg daily, rifampin 600 mg daily, ethambutol 1200 mg daily and pyrazinamide 2000 mg daily, vitamin B6 300 mg daily. His chronic medications were: enalapril 20 mg daily, amlodipine 5 mg daily, ticlopidine 500 mg daily. Therefore plasma concentrations of anti TB drugs were measured after seven days of drug intake in the hospital. Plasma samples were collected before the drug intake to measure Ctrough and after 2 hours to measure Cmax (peak plasma concentration). We found very low Cmax and AUC0-24 levels of isoniazid (0,395 mg\L and 4.75 hr*mg/L respectively), not attributable to patient's anti-hypertensive and anti-tubercular drug-drug interaction. The results of plasma concentrations, AUC0-24 and the therapeutic expected range are shown in the following table. The patient obtained slow clinical improvement.