William James Giesen, M.D.
Internal Medicine Physician · Darien, CT
William James Giesen, M.D. is an internal medicine physician with a practice address in Darien, Connecticut, and NPI 1124104724, issued on Oct 27, 2006. Check the license (Connecticut CT023721, as entered in NPPES) on the Connecticut Medical Examining Board; the Medicare figures below come from public CMS files.
- Name
- William James Giesen
- Credential
- M.D.
- Primary taxonomy
- Internal Medicine Physician 207R00000X
- License number
- CT023721 CT
Self-reported in NPPES; status is on the state search - Practice location
- 1500 Post RD., Darien, CT 06820
- Phone
- (203) 655-2516
- Enumerated
- Oct 27, 2006
- Record last updated
- May 9, 2016 (not updated in 10 years: call before relying on the address or phone)
- Fax
- (203) 656-3665
- Medicare enrollment
- Enrolled (PECOS I20031203000549); accepts Medicare assignment
- Medical or professional school
- Georgetown University School Of Medicine, 1975
- Entity
- Type 1 · individual
8 nearest internal medicine physician records → All 2,623 in Connecticut → Look up another NPI →
Verify William James Giesen in five checks
- 1IdentityNPI record in CMS NPPESNPI lookupConfirmed in NPPES
- 2LicenseActive license and public ordersConnecticut Medical Examining Board
Search for license CT023721Check the board - 3Other statesLicenses and actions elsewhereDocInfo (FSMB)Not checked
- 4ExclusionsMedicare and Medicaid bansHHS-OIG exclusion listNot checked
- 5CertificationBoard certificationABMS AOA NCCPA AANPNot checked
50% fewer Medicare Part D claims than the average internal medicine physician in 2024
1,595 Medicare Part D claims for 165 beneficiaries, against an average of 3,176 among the 108,855 providers of this specialty who prescribed in Part D.
Most prescribed
- Atorvastatin Calcium256 claims
- Amlodipine Besylate152 claims
- Losartan Potassium152 claims
- Hydrochlorothiazide98 claims
- Zolpidem Tartrate80 claims
- Escitalopram Oxalate69 claims
Source: CMS Medicare Part D Prescribers by Provider and Drug, 2024 (data.cms.gov). Part D covers prescriptions for Medicare patients only; drugs with fewer than 11 claims are not published.
William James Giesen among 2,623 internal medicine physician records in Connecticut
William James Giesen, M.D. is one of 2,623 internal medicine physician records with a practice address in Connecticut. 81% of them appear in a Medicare file (Part D prescribing, MIPS, Open Payments or Medicare group billing); this one does. Its NPI, issued in 2006, is newer than 11% of them.
- NPI issued2006 · 11% of these records are older, 52% newer
- In a Medicare fileYes · 81% of these records are
- In the CMS Doctors and Clinicians fileYes · 63% of these records are
- Graduated1975 · median 2004 among the 1,646 with a school on file
- Record last updated2016 · 61% of these records were not updated in five years or more
Source: CMS NPPES, file of Aug 9, 2026, every record with primary taxonomy 207R00000X and a Connecticut practice address; Medicare files: CMS Part D Prescribers 2024, MIPS 2023, Open Payments 2024, Doctors and Clinicians (data.cms.gov).
15 internal medicine physician records list a practice address in Darien
They are 15 of the 380 NPPES records with a practice address in Darien, and 2,623 internal medicine physician records in Connecticut.
All 2,623 in Connecticut → Every provider in Darien → Internal Medicine Physician nationwide →
The 8 internal medicine physician records with the nearest ZIP codes
- Gene Balboa, MDDarien, CT 068201841366820
- Margaret BaranDarien, CT 068201285623843
- Susan Collins, M.D.Darien, CT 068201306877626
- Amanda Collins- Baine, M.D.Darien, CT 068201154340040
- Mark Dam, M.DDarien, CT 068201720005507
- James Dorr, M.D.Darien, CT 068201427367705
- Karthik GunachelvanDarien, CT 068201740604941
- Carolyn Langelier, M.D.Darien, CT 068201780930933
Common questions
What is William James Giesen, M.D.'s NPI number?
William James Giesen, M.D.'s National Provider Identifier (NPI) is 1124104724, issued on Oct 27, 2006. An NPI identifies a provider in billing; it is not a license.
Where is William James Giesen, M.D. located?
The NPPES record lists a practice address at 1500 Post RD., Darien, CT, 06820. Phone: (203) 655-2516. Providers update this record themselves, so call before visiting.
How can I verify William James Giesen, M.D.'s license?
NPPES lists Connecticut license number CT023721, as entered by the provider. Search the Connecticut Medical Examining Board at https://www.elicense.ct.gov/Lookup/LicenseLookup.aspx. The NPPES record confirms identity only; the state search shows license status, expiration and public orders.
How much does William James Giesen, M.D. prescribe under Medicare Part D?
In 2024, William James Giesen, M.D. had 1,595 Medicare Part D claims for 165 beneficiaries; the average among internal medicine physician prescribers was 3,176. Part D covers Medicare prescriptions only.
Source: CMS National Plan and Provider Enumeration System (NPPES), file of Aug 9, 2026 (NPPES files on CMS.gov); NUCC Health Care Provider Taxonomy. Medicare files from data.cms.gov. An NPI identifies who bills; it is not a license.
Figures on PlainDoctor are read from the federal files each page names (CMS NPPES, Medicare Part D, MIPS, Open Payments; the HHS-OIG exclusion list only as counts) and from Census population estimates, with the file date shown beside them. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error. Data current as of Aug 9, 2026.