Vitamin supplementation is usually not necessary if you eat a balanced diet but can be considered if your diet is not as good as it could be. You can also consider getting an influenza vaccine every year, try to avoid crowded and enclosed areas during peak seasons like now, wash or sanitize your hands frequently especially before touching your eyes, nose and mouth, avoid over exerting yourself, get enough rest and sleep, drink adequate amount of fluids, eat a well balanced diet with plenty of fresh fruits and vegetables, avoid smoking or second hand smoke, and exercise regularly to improve your immune function.
Intestinal problems are uncommon side effects of Telfast, but it's possible. The best way to find out is to stop the medication for 2 weeks to see if the symptoms would go away. If the symptoms go away, you may need to talk to your doctor about trying a different allergy medicine.