I have used the dry[capsule] powdered extract as a general immune enhancer and it worked well for me. The key is to use reputable brands that prepare the product properly. I particularly like Source Naturals, Enzymatic Therapy, Now Foods and the Canadian company Natural Factors. In the liquid extract form the companies Herb Pharm and Gaia have excellent reputations for quality control- although I’m not sure if they do a nasal spray form or not off the top of my head. [they use a liquid dropper style for most products]
Another excellent nasal spray product for colds/flu and sinus problems is Source Naturals colloidal silver. And lots of Vitamin C etc. helps. Check out Bill Sardi’s writing at Rockwell’s site - he seems to know his stuff.