I answer another question about Seretide regarding potential side effects. As with most inhalers, Seretide Diskus has very few side effects which many times also result from taking the inhaler incorrectly (poor inhaler technique).
Most inhalers actually are very safe, with very rare adverse effects due to the low amount of medication they contain. Because the medication directly reaches the airways where it needs to act, you don’t require a lot of it to control potentially severe conditions. This dramatically improves safety,
One thing I would always suggest is to remember to rinse your mouth after using most inhalers. This can be done even with just water, by gargling some water and then spitting it out.