All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
Working as a physician or a pharmacist without having a certificate that reflects competence in those fields, and without having a licensed clinic or drug store, is prohibited. Not only that, but it also involves deceiving Muslims since it could do them harm or could, sometimes, lead to death. In fact, people working in those fields should fear Allah in patients through dedication and mastery of their jobs. However, practicing those professions without having acknowledged certificates is a clear sin regarding which the Prophet (PBUH) said: "He who deceives is not of us." {Muslim}.
In addition, impersonating others and pretending to have specialized knowledge is an act of deception. Furthermore, any harm that befalls any Muslim as a result renders such physician or pharmacist liable because the Prophet (PBUH) said: "Whoever practices medicine when he is not known for that, he is liable" {Abu Dawoud}. And Allah Knows Best.