Doxoril 400mg Tablet is used to treat and prevent symptoms of asthma and chronic obstructive pulmonary disorder (a lung disorder in which flow of air to the lung is blocked). It helps in relaxing the muscles of the air passages, thus widening it and making it easier to breathe. Doxoril 400mg Tablet can be taken with the food in the evening. Take it in the exact dose and duration as prescribed by your doctor. The dose and how often should take it may depend on what you are taking it for. Take it for as long as it is prescribed for you. In case you missed a dose, take it as soon as you remember it. It does not work immediately or prevent the already started breathing trouble, hence always carry your medical inhaler with you for an emergency. The most common side effects of this medicine include vomiting, headache, nausea, and stomach upset. If these bother you or appear serious, let your doctor know. There may be ways of reducing or preventing them. Generally, it is advised not to consume alcohol or do not smoke while on treatment. You must avoid high caffeinated products like coffee, tea, and dark chocolates as it enhances drug side effects. Before taking this medicine, let your doctor if you have kidney or liver disease or if you have heart-related problems. Your doctor should also know about all other medicines you are taking as many of these may make this medicine less effective or change the way it works. Inform your doctor if you are pregnant or breastfeeding before starting the treatment.