Giardia is a protozoal parasite found in lakes, streams, ditches and other outdoor water sources. The parasite is shed by wild animals high up in the watershed, and as the water travels down, it becomes even more contaminated by other animals living near it. Any dog or cat drinking from a pond, river, or other natural water source is likely to ingest the cysts of this persistent parasite. Once inside, the cysts hatch, grow, and reproduce. Giardia causes profuse, uncontrollable, often watery diarrhea, sometimes with blood and mucus associated with it.
The drugs most commonly used to treat Giardia are metronidazole, an antibiotic, and fenbendazole a dewormer. Each of them is about 70% effective; that is, they only work about 70% of the time. Because some parasites may be left alive, recurrent infections are possible even after treatment.
However, there is a natural protocol for getting rid of Giardia, that has been used many times successfully (and safely) in cats and dogs. We even cleared a cattery that had many infected kittens. It utilizes digestive enzymes on an empty stomach. With no food in the system, the digestive enzymes go to work on anything else that might be in the intestines--including parasites. It's been reported to work for Coccidia, and could also theoretically get rid of Toxoplasma, Cryptosporidia, Neosporum, Tritrichomonas foetus, and similar protozoal parasites.
First, you'll need a bottle of digestive enzymes (plant or fungi based) and one or more syringes; you can usually talk your vet into giving you a few 3 or 6 cc syringes (without needles). Do not get enzymes in combination with anything else such as probiotics or other supplements. It must be pure digestive enzymes alone. Make sure the product you select contains at least protease, amylase, and lipase (some also contain cellulase and other enzymes). These digest protein, starch, and fat, respectively. One product I have used is called Prozyme, which is available at many pet stores and online.
Note that the enzymes MUST be given on an empty stomach. If you leave food out for your pet 24/7, this protocol will NOT work. Food must be fed in meals, at least 30 (preferably 60) minutes after each dose of enzymes. If there is any food in the animal's digestive system, the enzymes will digest the food instead, and the parasites will flourish undisturbed. enzymes cannot be hidden in a pill pocket or a piece of cheese--there can be no food in the tummy!
Take 1 capsule of enzymes for each animal to be treated, and mix with a tiny amount of water to make a slurry. Use the syringe to give the slurry by mouth. In most cases it's easiest to insert the syringe in the corner of the mouth between or behind teeth and squirt. The mixture does not taste that bad, but cats in particular are just not gracious about taking meds. Be careful, but be firm.
Round 1: Give a dose of digestive enzymes 1/2 to 1 hour before morning and evening meal, and a dose at bedtime. Take the syringe apart and rinse clean after each dose (otherwise the residue builds up and the plunger will stick). Repeat daily for 8 days.
Rest: Take a break for 1 week. This allows time for resistant cysts to "hatch."
Round 2: Repeat digestive enzyme treatment for 7 days.
And that's it! In my experience in 15 years as a veterinarian, this protocol has been a much safer and more effective treatment than drugs.