Is diphenhydramine (Benadryl) really dangerous for a child with asthma?

You will notice this mention on all cough syrups. Indeed, if an asthmatic has an asthma attack, his recovery depends on his cough reflex. And note that many asthmatics also have allergies and Benadryl can save their lives! So, it's very much a case by case basis and many pharmacists could qualify their answer differently

One thing is certain, the priority when a cough starts in an asthmatic is the pumps. An action plan can be discussed with your healthcare professional (pharmacist or pediatrician) to determine when it is optimal to double your child's dose of Alvesco for a few days. Salbutamol can be used for coughing spells up to 4 times a day. At your child's age, cleaning the nose (even if the child doesn't always like it) is a really important step up to 4 times a day. For children over 6 years old whose asthma is very well controlled, I sometimes give syrup at bedtime if the cough prevents them from sleeping. For a child of your age, there are also natural or homeopathic products that are safe to take at bedtime only, but there are no conclusive studies showing their effectiveness

In short, ideally we rinse the nose and optimize the pumps for the time being. I hope this is a guide for you

Also, don't blame yourself for the dose of Benadryl, the doses on the box are generally very low too and the risk unless he had an asthma attack recently, all in all, quite low.