Islatravir and Methadone Pharmacokinetics (MK-8591-029)

Inclusion Criteria: * Has a body mass index (BMI) \> 18 and ≤ 35 kg/m\^2 * Is in good health based on laboratory safety tests obtained at the screening visit and prior to administration of study drug * Is in good health based on medical history, physical examination, vital sign measurements, and electrocardiograms (ECGs) performed prior to randomization. * Has a negative human immunodeficiency virus (HIV) antigen/antibody test at screening * For male participants, follows contraception guidance consistent with local regulations * For female participants: * Is not a woman of childbearing potential (WOCBP) or * Is a WOCBP and using acceptable contraception or is abstinent * Is reliably participating in a methadone maintenance program for at least two (2) months prior to Day 1 * Agrees to not change their current maintenance methadone dose of 20-200 mg administered as a single daily dose Exclusion Criteria: * Has a history of clinically significant endocrine, gastrointestinal, cardiovascular, hematological, hepatic, immunological, renal, respiratory, genitourinary, or major neurological (including stroke and chronic seizures) abnormalities or diseases * Is mentally or legally incapacitated, has significant emotional problems at the time of prestudy (screening) visit or expected during the conduct of the study or has a history of clinically significant psychiatric disorder of the last 5 years * Has a history of cancer (malignancy) * Has a history of significant multiple and/or severe allergies (eg, food, drug, latex) or has had an anaphylactic reaction or significant intolerability (ie, systemic allergic reaction) to prescription or non-prescription drugs or food * Had major surgery, donated or lost 1 unit of blood (approximately 500 mL) within 4 weeks prior to the screening visit * With the exception of methadone, is unable to refrain from or anticipates the use of any medication, including prescription and non-prescription drugs or herbal remedies beginning approximately 2 weeks (or 5 half-lives) prior to the first dose of the 14-day methadone maintenance run-in phase prior to Day 1, throughout the trial, until the AE follow-up call (Day 16) * Has participated in another investigational study within 4 weeks (or 5 half-lives) prior to the prestudy (screening) visit. * Has a QTc interval \>450 msec (males) or \>470 msec (females), has a history of risk factors for Torsades de Pointes (eg, heart failure/cardiomyopathy or family history of long QT syndrome), has uncorrected hypokalemia or hypomagnesemia, is taking concomitant medications that prolong the QT/QTc interval other than methadone * Does not limit smoking to no more than 10 cigarettes per day while in the clinical research unit (CRU) * Consumes greater than 3 glasses of alcoholic beverages per day * Consumes excessive amounts, defined as greater than 6 servings (1 serving is approximately equivalent to 120 mg of caffeine) of coffee, tea, cola, energy drinks, or other caffeinated beverages per day * With the exception of tetrahydrocannabinol (THC), has a positive screen for drugs with a high potential for abuse such as cocaine, amphetamines, methylenedioxymethamphetamine (MDMA), barbiturates, benzodiazepines (with the exception noted in exclusion criteria 7), or opiates/opioids on Day -1 * Presents any concern by the investigator regarding safe participation in the study or for any other reason the investigator considers the participant inappropriate for participation in the study