I Have Had The Flu (had Flu Injection Last April

I have had the flu (had flu injection last April & took buccaline Berna tablets 21/2 weeks ago) for the past 2 weeks with a severe chesty cough, that worsens at night, lemon/honey/thyme infusions and hot drinks give temporary relief through the day; I am thinking of buying some Benadryl Forte cough mixture from my chemist to get up the thick greenish brown phlegm. however I had a cardio ablation 6 weeks ago; is this mixture OK for me?