Co-Telsan 40/12.5mg tablets are a combination medication used to treat high blood pressure. Each tablet contains two active ingredients: telmisartan (40mg) and hydrochlorothiazide (12.5mg). Telmisartan belongs to a class of drugs called angiotensin II receptor blockers (ARBs), which work by relaxing blood vessels, while hydrochlorothiazide is a diuretic that helps your body get rid of excess salt and water to lower blood pressure. Co-Telsan tablets are usually taken once a day, with or without food, as directed by your doctor. Common side effects may include dizziness, headache, and stomach upset.