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Man-Siak Mak, MD

NPI 1609875004ActiveIndividual· Internal Medicine, Gastroenterology· Merced, CA

Internal Medicine, Gastroenterology (MD) with Timothy S Johnston MD PC, practicing in Merced, California. In 2024 Medicare paid $52,358 for 178 patients, and Part D covered 219 of their prescriptions, 84% of them generic. Lists Mercy Medical Center as a hospital affiliation.

Overview

NPI1609875004
StatusActive
Entity typeIndividual
CredentialMD
Sole proprietorYes
NPI issuedJul 18, 2005
Last updated in NPPESApr 17, 2019
ReactivatedMar 21, 2006

Specialties and licenses

TaxonomyCodeLicenseState
Internal Medicine, Gastroenterology Primary207RG0100XA26785CA

About license numbers in NPPES

Exclusion screening

ListResultDetails
HHS-OIG LEIE · Nov 10, 2026No matchChecked by NPI and by name + state
North Carolina Medicaid · Nov 11, 2026No match—
New York OMIG · Nov 11, 2026No match—
Ohio Medicaid · Nov 11, 2026No match—
Pennsylvania Medicheck · Nov 11, 2026No match—
Washington HCA · Nov 11, 2026No match—

Official OIG search · How matching works ·Bulk check a roster

Medicare

Enrollment (PECOS)Enrolled Practitioner - Gastroenterology (CA)
Accepts assignmentYes (participating)

Medicare services in 2024

Medicare patients178
Services billed533 across 19 procedure codes
Paid by Medicare$52,358

Part D prescriptions in 2024

Prescriptions (claims)219 · 337 30-day fills
Patients102
Total drug cost$17,831
Generic share84% of claims

Medicare sources and data years

Education and affiliations

Graduation year1968
Medicare specialtyGastroenterology
Secondary specialtiesInternal Medicine

Medical group

Hospital affiliation

Source: CMS Care Compare

Industry payments (Open Payments)

None reported for 2025 or 2024.

Receiving payments does not imply wrongdoing. What Open Payments covers ·original records

Other gastroenterologists in Merced

All gastroenterologists in California