I'm responding to Kat 58 in Montreal, who reports that since her COPD diagnosis last summer, her doctor has switched her from Symbicort to Incruse to Anoro then to Breo without discussing the rationale. Is Trelegy next?
Despite the dozens of brand names (which differ somewhat amongst countries), there are only 3 types of drugs in asthma/COPD inhalers: LABA, LAMA, and ICS. The first two are bronchodilators which relax the smooth muscles which surround airways. The third is like applying cortisone cream to a skin rash, it reduces inflammation. Each has its own set of side effects, some serious. Doctors cannot predict which individual patient benefit from the drug, will experience a side effect (or both). Only you will know by taking that drug. If you are taking a dual LABA+LAMA COPD inhaler, or one of the new triple drug inhalers, it is only a guess which drug is causing the side effect which you experience.
The only "black box" warning given by the FDA for inhalers is that if you only have asthma, your risk of a fatal asthma attack is substantially increased if you only take a LABA regularly without an ICS. That is why LABA+ICS inhalers were then prescribed for almost all patients with asthma exacerbations. However, it became unusual for a doctor to "step down" to an ICS only inhaler after a few months of asthma control.
ICS drugs are great for suppressing the eosinophilic airway inflammation of allergic asthma, but in adult smokers, they increase the risk of a community-acquired pneumonia (aka CAP) and candida (aka thrush) because they decrease the ability of white cells (neutrophils and lymphocytes) to fight bacterial and fungal infections.
The GOLDen clinical practice guidelines for COPD were largely written by key opinion leaders (KOLs) assisted by "ghost writers" directly or indirectly supported by inhaler manufacturers. They have not yet addressed the value of "stepping down" doses or the number of drugs when the inhalers are ineffective for an individual patient, or cause harm.
Disclaimer: Although I'm a pulmonary specialist, you must discuss any changes in your COPD therapy with YOUR pulmonary specialist.