Licensed dentists and members of good standing Philippine Dental Association, are exempted from payment of Mayor’s Permit because the practice of Dentistry is an exercise of a profession and not a business concern.

(based on Certification issued by Clodualdo C. De Jesus, De Jesus and Associates Law Office on March 10, 2008 in Pasig City)

This is pursuant to the prevailing ruling of then Finance Secretary Edgardo D. Espiritu on December 16, 1999 Annex A, which states:
However, a person engaged in the practice of a profession requiring government examination such as a dentist, is, by express provision of the Local Government Code, exempt from this requirement. The code instead provides for a professional tax on professionals belonging to this class. Such professional who has paid the corresponding professional tax to the province where he practices his profession shall be entitled to practice his profession in any part of the Philippines without being subjected to any other national or local tax, licenses, or fee, including the Mayor’s Permit or license fee, for the practice of such profession.

According to page 226 of the Local Government Code of 1991, annotated, 1st Edition:

Payment of the professional tax (PTR) herein will preclude imposition of any other tax or fee by any other local government unit for the exercise of one’s profession.
SEC. 139. Professional Tax. – (b) Every person legally authorized to practice his profession shall pay the professional tax to the province where he practices his profession or where he maintains his principal office in case he practices his profession in several places: Provided, however, That such person who has paid the corresponding professional tax shall be entitled to practice his profession in any part of the Philippines without being subjected to any other national or local tax, license, or fee for the practice of such profession.

also, section 147 of RA 7160:

Section 147. Fees and Charges. – The municipality may impose and collect such reasonable fees and charges on business and occupation and, except as reserved to the province in Section 139 of this Code, on the practice of any profession or calling, commensurate with the cost of regulation, inspection and licensing before any person may engage in such business or occupation, or practice such profession or calling.

also sec 3, par (e) of RA 3019

Section 3. Corrupt practices of public officers. In addition to acts or omissions of public officers already penalized by existing law, the following shall constitute corrupt practices of any public officer and are hereby declared to be unlawful:
(e) Causing any undue injury to any party, including the Government, or giving any private party any unwarranted benefits, advantage or preference in the discharge of his official administrative or judicial functions through manifest partiality, evident bad faith or gross inexcusable negligence. This provision shall apply to officers and employees of offices or government corporations charged with the grant of licenses or permits or other concessions.