What you may be referring to is the updated safety information that was required to be dispensed with Advair (Serevent is a component of it). In 2005 the product was required to carry a stronger warning stating that there is a risk of more severe asthma episodes when using it. The product IS safe. All medications are required to report any occurring adverse reactions (even if there is a very minimal risk of that occuring.) In November of 2005, the FDA required stronger labels on that particular medication.
I try not to get too worried about side effects unless it's something I know has been causing me problems in the past. Here in the United States people are so eager to want to litigate that big companies are forced to protect themselves from liability by labeling their products a certain way. It's the same reason why you see those seemingly ridiculous warnings on your hair dryer like "Do not use while sleeping."
If you're still concerned about the safety of the product, ask your pharmacist or doctor. Your doc wouldn't have prescribed it for you if he didn't think the benefits outweigh the risks of this particular medication. Keep in mind that Serevent, Foradil, and Advair are ASTHMA medications that are used to control and prevent bronchospasm. They are not a CF targeted medication, but doctors have seen this medication work for our symptoms. Remember too that Advair is not to be used as a "rescue med" in the event of an asthma attack. When taking it as directed and letting your doctor and pharmacist know what other medications you are taking, the chances of having an adverse reaction is reduced.