Phenergan Syrup should be taken with or without food, but take it at the same time every day to get the most benefit. 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. you must inform your doctor if symptoms does not get improve within 7 days of treatment.