The medical treatment for Giardia is usually a drug called metronidazole. A recent study showed that taking milk thistle concurrently with metronidazole reuslted in much better efficacy against Giardia.
Herbs with high berberine levels like Barberry or Oregon grape can be effective against protozoal infections as can whole garlic and chaparral.
Black walnut is good for other internal parasites but I haven’t used it for Giardia. No problem with extended use so long as one take a day off once a week or so. I don’t know how effective it is against Giardia.
Doc
Don't use herbs or combine herbs with medications or use them during lactation or pregnancy without talking with your healthcare provider.