if you are young and not really suffering from serious ED. I'd try 5mg.
A FULL dose is 20mg, that will likely make you stuffy, give you a headache, etc.. 10mg will make you have just as hard wood but less sides.. 5mg does the same for me. Some people can get by with 2.5mg.. no joke.
But with that liquid stuff I'm not sure if those dosages will all be the same. In theory they "SHOULD" be, but who knows. I always use prescription tabs. My insurance will cover 8 tabs a month for me which ends up being 32 doses!