At the rescue group I volunteered with (this was near Atlanta, so giardia strains may be different depending on region), we tended to have much better success treating giardia with metronidazole (flagyl) than panacur, although panacur was more mild and a little safer for young kittens. It seems like it's a similar medication to Secnidazole, but we did have to give it daily for a week.