 In Step 1, participants will receive one of four different doses of NTZ. Participants will take NTZ twice a day for 56 days in either a liquid or pill form. All participants will be closely monitored for drug toxicity. There will be seven study visits; they will occur at study entry, Weeks 1, 2, 4, 6, and 8, and Day 70. Study visits will include a physical exam and blood, urine, and stool collection. Pharmacokinetic PK sampling will be performed during four of the study visits. PK sampling requires the participants to take their morning NTZ doses while in the clinic; participants will undergo additional blood collection either before or after taking NTZ. At the end of the 56-day study period, participants who are experiencing a positive clinical benefit from NTZ and who have had no harmful side effects may choose to continue taking NTZ for an additional 24 weeks and enter Step 2. Participants who do not continue taking NTZ after Day 56 will be followed for 2 additional weeks. 