Q: Our 10 1/2-year-old female cockapoo has emphysema and an allergy . She is always coughing and has trouble breathing. Vitamin C seems to help her. What dosage do you recommend? Are there side effects?--M.S. A: It sounds as if your dog suffers from respiratory allergies ranging from rhinitis (nasal inflammation), marked by sneezing, to emphysema, which is a serious sequela of chronic asthma. I am not aware of any controlled studies that substantiate the effectiveness of Vitamin C in controlling such problems in dogs. Emphysema is an irreversible loss of lung elasticity that causes breathing distress because air can't be effectively expelled from the lungs. Rest, moderate temperatures, moderate humidity and a dust-free environment are the best conditions you can provide. Your veterinarian might prescribe antihistamines, bronchodilators or corticosteroids for the allergic symptoms. If you feel Vitamin C helps your dog, continue giving it to her. For a small dog, I'd suggest 250 milligrams daily. It's not toxic; what is not utilized is eliminated, if the kidneys are functioning properly. Be aware, though, that dogs synthesize their own Vitamin C in adequate amounts.