What dosage?

I don't know much; this here comes from my own experiences with Cortef. Perhaps someone else can tell you more. The following links also provide reputable drug information and list other drug interactions:
http://www.drugs.com http://www.rxlist.com/

Very low dose corticosteroids such as Cortef (around 20 mg/day) are used by some docs to support the adrenals. Such low doses are considered physiological and not pharmacological, and do not seem to suppress the immune system.

Higher doses, particularly over 40 mg (and this depends on the steroid -- I believe steroids like Prednisone have a much lower threshold for pharmacological doses) can supress the immune system, and are Not a Good Thing for Lyme patients ( or anyone with an active infection).

In addition, steroids can really aggravate a lyme rage, and contribute to the general feelings of anxiety and antsyness.

Here is one of the manufacturer's warnings on Nasonex: