Histacin is an antiallergic medication used in the treatment of various allergic conditions. It provides relief from runny nose, sneezing, itching and watery eyes. Histacin should be taken with or without food, but take it at the same time every day to get the most benefit. It should be taken as your doctor's advice. The dose and how often you take it depends on what you are taking it for. Your doctor will decide how much you need to improve your symptoms. You should take this medicine for as long as it is prescribed for you. In case, you have missed any doses than it is better to take the next dose as soon as you remember it. However, you should never take a double dose. Sleepiness or drowsiness is the most common side effect of this medicine. Avoid driving or attention-seeking activity. If these bother you or appear serious, let your doctor know. There may be ways of reducing or preventing them. In some cases, it may cause dryness of mouth, so it is advised to drink more water, always carry sugar candy or maintain oral hygiene. Before taking this medicine, let your doctor know if you have glaucoma or have any gastro related issues or have asthma. Inform your doctor if you are pregnant, or breastfeeding. 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. Generally, it is advised to avoid alcohol while on treatment.