Dr. Gary Maassen, MD
- Medicare services, 2023
- 1,227
- Part D claims, 2024
- 3,009
- Industry payments
- $6,004
- Affiliated sites
- 1
- Medical group
- 1
- Practice address
- 5034 Griffin Rd, Saint Louis, MO 63128-3418
- Practice phone
- Call (314) 843-7333
- Professional email
- Get professional email
- Personal email
- Get personal email
- Mobile number
- Get mobile number
Primary practice
Esse Health
Primary care practice · Saint Louis, Missouri
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Dr. Gary Maassen is an internist in Saint Louis, Missouri
Dr. Maassen practices at Esse Health in Saint Louis, Missouri.
Dr. Maassen's NPI, 1831170968, was assigned on November 7, 2005. Dr. Maassen graduated from Saint Louis University School of Medicine in 1986. Dr. Maassen holds Missouri license R7H60.
- NPI
- 1831170968NPI Registry (opens in a new tab)
- NPI enumeration date
- November 7, 2005
- NPI status
- Active
- Practice phone
- (314) 843-7333
- Professional email
- Get email
- Personal email
- Get email
- Mobile number
- Get number
- Sex
- Male
- Estimated age
- 66
- Medical school graduation
- 1986
Sources: NPPES NPI Registry (CMS); Dmand AI Professional Profile Data; Dmand AI Affiliation Data; and Doctors and Clinicians National Downloadable File (CMS)
Dr. Gary Maassen practices at 2 locations in Saint Louis, Missouri
Dr. Maassen's primary practice is Esse Health, at 5034 Griffin Rd, Saint Louis, MO 63128-3418. The practice phone is (314) 843-7333. The NPI Registry lists Dr. Maassen's practice address as 1 Barnes Jewish Hospital Plz, Saint Louis, MO 63110.
- Esse HealthPrimary5034 Griffin Rd, Saint Louis, MO 63128-3418Phone(314) 843-7333
- NPI Registry address1 Barnes Jewish Hospital Plz, Saint Louis, MO 63110 · Saint Louis CityPhone(314) 843-7333
Sources: NPPES NPI Registry (CMS); Dmand AI Affiliation Data; Doctors and Clinicians National Downloadable File (CMS); and Dmand AI Practice Data
Dr. Gary Maassen is affiliated with 1 site of care and 1 medical group
Dr. Maassen's primary practice is Esse Health in Saint Louis, Missouri. The medical group on file is Esse Health.
Sources: Dmand AI Affiliation Data; and Doctors and Clinicians National Downloadable File (CMS)
Dmand AI tracks the EHR system at Dr. Gary Maassen's practice
Dmand AI tracks the electronic health record (EHR) system at each site where Dr. Maassen practices. Vendor and product names are available with a free account.
| Site | EHR vendor and product |
|---|---|
| Esse Health | Available with a free account |
Source: Dmand AI Technology Install Data
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Dr. Gary Maassen is one of 2,114 internists in Missouri
Internal medicine is Dr. Maassen's primary specialty in the NPI Registry, under taxonomy code 207R00000X. Dr. Maassen is licensed in Missouri.
Specialties and state licenses
- Internal medicine, MissouriPrimary
Internists near Dr. Maassen
Counts of internists with a Dmand AI profile, by place and practice. Each opens the list.
Sources: NPPES NPI Registry (CMS); Dmand AI Professional Profile Data; and Dmand AI Peer Benchmarks
Dr. Gary Maassen billed Medicare for 1,227 services in 2023
CMS data shows 1,227 Medicare services from Dr. Maassen across 20 procedure codes in 2023. Medicare paid $40,284 for them. Every service was in an office.
Dr. Maassen's Medicare services by type, 2022 to 2023
- 2022
- 2023
- Lab and pathology
- 2022: 7832023: 54131%
- Surgery
- 2022: 3552023: 27523%
- Office and hospital visits
- 2022: 3112023: 26116%
- Other services
- 2022: 1192023: 9322%
- Tests and treatments
- 2022: 612023: 577%
- Imaging
- 2022: 142023: 0100%
Dr. Maassen compared with the Missouri median for internists, 2023
- Dr. Maassen
- Missouri median
- Medicare services
- 1,227vs 5302.3 times
- Part D claims
- 2,482vs 3257.6 times
Medians across 2,076 internists in Missouri with Dmand AI profiles that report each measure.
Dr. Maassen's most billed Medicare procedure codes, 2023
CMS publishes codes billed for 11 or more Medicare patients. Each label is the code family; CPT descriptions are licensed by the AMA.
Need commercial claims for Dr. Maassen?
Medicare shows one payer. Book a meeting to get commercial claims data for Dr. Maassen and other internists in Missouri.
Sources: Medicare Physician & Other Practitioners - by Provider and Service (CMS), 2024 data; and Dmand AI Peer Benchmarks
Dr. Gary Maassen's patients have a median age of 71
Across all payers, Dmand AI patient panel data puts Dr. Maassen's median patient age at 71, with 45% women.
Dr. Maassen's patients across all payers
Dmand AI patient panel data covers Medicare, Medicaid and commercial patients.
Patient panel by age, all payers
Median age 71
Patient panel by sex, all payers
- Women45%
- Men55%
Source: Dmand AI Patient Panel Data
Dr. Gary Maassen wrote 3,009 Medicare Part D claims in 2024
In 2024, Dr. Maassen wrote 3,009 Medicare Part D claims for 440 patients. 85% of them were for generic drugs. CMS names the drug for 2,482 of them, across 64 drugs with 11 or more claims each, at $232,349 in total drug cost. In 2023, CMS named 10,123 claims across 149 drugs.
Dr. Maassen's Part D claims by kind of drug, 2024
- Generic2,556 claims85%
- Brand-name430 claims14%
- Other23 claims1%
Dr. Maassen's most prescribed drugs by claims, 2024
CMS names a drug when it has 11 or more claims. Share is of the claims for drugs CMS names.
Drug classes CMS tracks in Dr. Maassen's Part D claims, 2024
- Antipsychotics, patients 65 and over1%22 of 3,009 claims
Share of all Part D claims for each drug class CMS reports on. Classes with no claims are left out.
Part D claims for drugs CMS names, 2023 to 2024
- 2023
- 2024
- Claims for drugs CMS names
- 2023: 10,1232024: 2,48275%
Drugs named by CMS: 64 in 2024, 149 in 2023.
Sources: Medicare Part D Prescribers - by Provider and Drug (CMS), 2024 data, released May 6, 2026; and Medicare Part D Prescribers - by Provider (CMS), 2024 data
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Dr. Gary Maassen received $6,004 in industry payments from 2016 to 2025
CMS Open Payments shows 414 general payments to Dr. Maassen from 31 drug and device companies from 2016 to 2025. Food and beverage make up the largest share, 99% of the total. Novo Nordisk Inc paid the most, $728.
General payments to Dr. Maassen by program year, 2016 to 2025
General payments to Dr. Maassen by kind, 2016 to 2025
- Food and beverage$5,93399%
- Education$711%
Companies that paid Dr. Maassen the most, 2016 to 2025
Share is of all general payments to this clinician.
Source: Open Payments (CMS), 2025 data, released July 7, 2026
Dr. Gary Maassen graduated from Saint Louis University School of Medicine
Dr. Maassen graduated from Saint Louis University School of Medicine in 1986.
- Medical school
Source: Dmand AI Professional Profile Data
Sources for Dr. Gary Maassen's profile data
| Data on this page | Source |
|---|---|
| NPI, name, sex, specialty and licenses | NPPES NPI Registry (CMS) (opens in a new tab)Dmand AI Professional Profile Data |
| Practice locations and phone | NPPES NPI Registry (CMS) (opens in a new tab)Dmand AI Affiliation DataDoctors and Clinicians National Downloadable File (CMS) (opens in a new tab)Dmand AI Practice Data |
| Primary practice, affiliations and health system | Dmand AI Affiliation DataDoctors and Clinicians National Downloadable File (CMS) (opens in a new tab) |
| Medicare services and procedure codes | Medicare Physician & Other Practitioners - by Provider and Service (CMS) (opens in a new tab) |
| State medians for the same specialty | Dmand AI Peer Benchmarks |
| Medicare patients and the all-payer patient panel | Dmand AI Patient Panel Data |
| Part D prescribing | Medicare Part D Prescribers - by Provider and Drug (CMS) (opens in a new tab)Medicare Part D Prescribers - by Provider (CMS) (opens in a new tab) |
| Industry payments | Open Payments (CMS) (opens in a new tab) |
| Training, board certification and memberships | Dmand AI Professional Profile Data |
| EHR at each practice site | Dmand AI Technology Install Data |
| Similar physicians | Dmand AI Affiliation Data |
Data last refreshed October 2026Report an inaccuracyRemove my information
FAQ
Frequently asked questions about Dr. Gary Maassen
Spotted something out of date? Email hello@dmand.ai and we will check the record. Contact details live in the free trial.
What is Dr. Gary Maassen's NPI number?
Dr. Gary Maassen's NPI number is 1831170968. It was assigned on November 7, 2005, and it is active in the NPI Registry.
What kind of doctor is Dr. Gary Maassen?
Dr. Gary Maassen is an internist in Saint Louis, Missouri. The primary specialty in the NPI Registry is Internal Medicine Physician (taxonomy code 207R00000X).
Where does Dr. Gary Maassen practice?
Dr. Maassen practices at 5034 Griffin Rd, Saint Louis, MO 63128-3418. Dr. Maassen also practices at 1 Barnes Jewish Hospital Plz, Saint Louis, MO 63110. Dr. Maassen's primary practice is Esse Health in Saint Louis, Missouri.
What is Dr. Gary Maassen's office phone number?
The practice phone on file for Dr. Maassen is (314) 843-7333.
What is Dr. Gary Maassen's medical license number?
Dr. Maassen holds Missouri license R7H60.
What medical group is Dr. Gary Maassen part of?
Dr. Maassen is linked to Esse Health.
Does Dr. Gary Maassen accept Medicare?
Yes. CMS data shows Dr. Maassen billed Medicare for 1,227 services in 2023.
What services does Dr. Gary Maassen bill to Medicare?
In 2023, Dr. Maassen's most billed Medicare services were surgery: heart and blood vessels (36415), office visit, established patient (99214) and lab and pathology test (80048).
Where did Dr. Gary Maassen go to medical school?
Dr. Maassen graduated from Saint Louis University School of Medicine in 1986.
What drugs does Dr. Gary Maassen prescribe most?
In 2024 Medicare Part D data, Dr. Maassen's most prescribed drugs by claims were Atorvastatin Calcium, Amlodipine Besylate and Metoprolol Succinate.
Has Dr. Gary Maassen received payments from drug or device companies?
CMS Open Payments shows $6,004 in general payments to Dr. Maassen from 31 companies from 2016 to 2025. The largest total came from Novo Nordisk Inc.
How long has Dr. Gary Maassen been practicing?
Dr. Maassen graduated from medical school in 1986, 40 years ago.
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