The purpose of this study is to investigate the potential pharmacokinetic (what the body does to the drug) interactions between multiple doses of phenytoin 200 mg every 12 hours or carbamazepine 200 mg every 12 hours and telaprevir 750 mg every 8 hours at steady-state (constant concentration of medication in the blood) in healthy participants.

This is a Phase I, open-label (all people know the identity of the intervention), randomized (the study medication is assigned by chance), 2-panel, sequential treatment study. In this study 24 healthy participants will be randomly assigned equally to 2 panels. In Panel 1 the participants will receive telaprevir in Part 1 (telaprevir 750 mg every 8 hours from Day 1 to Day 9 followed by a single 750 mg dose in the morning on Day 10) and phenytoin/telaprevir in Part 2 (phenytoin 200 mg every 12 hours from Day 1 to Day 16 followed by a single 200-mg dose in the morning on Day 17; and telaprevir 750 mg every 8 hours from Day 8 to Day 16 followed by a single 750 mg dose in the morning on Day 17). In Panel 2 the participants will receive telaprevir (telaprevir 750 mg every 8 hours from Day 1 to Day 9 followed by a single 750 mg dose in the morning on Day 10) and carbamazepine/ telaprevir (carbamazepine 200 mg every 12 hours from Day 1 to Day 16 followed by a single 200-mg dose in the morning on Day 17; and telaprevir 750 mg every 8 hours from Day 8 to Day 16 followed by a single 200-mg dose in the morning on Day 17). In both panels, Part 1 and Part 2 are separated by a washout period of at least 2 weeks and maximum 4 weeks. Safety and tolerability will be assessed by evaluating adverse events, electrocardiogram, clinical laboratory examinations, vital signs and physical examination throughout the study period.

Inclusion Criteria:

• Must be healthy on the basis of physical examination, medical history, vital signs, clinical laboratory tests, and electrocardiogram performed at screening
• If a woman, before entry she must be postmenopausal for at least 2 years (amenorrheal for at least 3 years), or surgically sterile (have had a total hysterectomy or bilateral oophorectomy, tubal ligation/bilateral tubal clips without reversal operation, or otherwise be incapable of becoming pregnant)
• Men must agree to use a highly effective method of birth control (ie, male condom with either female intrauterine device [IUD], diaphragm, cervical cap or hormone based contraceptives by their female partner) and to not donate sperm during the study and for 3 months after receiving the last dose of study drug
• A 12-lead electrocardiogram consistent with normal cardiac conduction and function

Exclusion Criteria:

• Participants with a history of any illness that, in the opinion of the Investigator or the participant's general practitioner, might confound the results of the study or pose an additional risk in administering study drug(s) to the participant
• Participants of Asian ancestry (given association of phenytoin / carbamazepine and severe rash with HLA-B 1502 in this population)
• Current use of prescription medication, and regular use or routine use of concomitant medication(s), including over-the-counter (OTC) products
• Consumption of herbal medications or dietary supplements (eg, St. John's Wort, Ginkgo biloba, garlic supplements), vitamins, and grapefruit or grapefruit juice, apple juice, or orange juice within 14 days before the first administration of study drug
• Consumption of an average of more than five 240-mL servings of coffee or other caffeinated beverage per day