Hello @jmkeough, Welcome to Connect. Here's some information I found on Mayo Clinic's site. I'm not sure if it's the same medication your doctors tried.

"What is the best treatment for Giardia?
Many people who do have problems often get better on their own in a few weeks. When signs and symptoms are severe or the infection persists, doctors usually treat giardia infection with medications such as: Metronidazole (Flagyl). Metronidazole is the most commonly used antibiotic for giardia infection. Jul 16, 2021"
--- Giardia infection (giardiasis) - Diagnosis & Treatment: https://www.mayoclinic.org/diseases-conditions/giardia-infection/diagnosis-treatment/drc-20372790

@irene5 and @athenalee have mentioned having giardiasis and may have some suggestions or information for you. Did the infectious disease specialist you saw start you on a specific treatment?