Around the age of thirty he was first placed in-charge of the first Royal Hospital at Ray, from where he left for Baghdad (now in Iraq), where he was active in the reconstruction of the city hospital, where he remained the head of its famous Muqtadari Hospital for along time.. Al-Razi became famous as the most prominent physician in the Islamic world, his fame comparable only to that of another Persian physician, Ibn Sina. The practical experience gained at the well-known Muqtadari Hospital helped him in his chosen profession of medicine. At an early age he gained eminence as an expert in medicine and alchemy, so that patients and students flocked to him from distant parts of Asia.
Razi was a Hakim, an alchemist and a philosopher. In medicine, his contribution was so significant that it can only be compared to that of Ibn Sina. Some of his written works in medicine have been widely studied, Latin editions of which remained in use as late as the seventeenth century in Europe, e.g. Kitab al- Mansoori (کتاب المنصوری), Al-Hawi (الحاوی), Kitab al-Mulooki (کتاب الملوکی)and Kitab al-Judari wa al- Hasabah(کتاب الجزداری و حسابہ) earned everlasting fame. Kitab al-Mansoori(کتاب المنصوری) , which was translated into Latin in the 15th century C.E., comprised ten volumes and dealt exhaustively with Greco-Arab medicine(ادویات العربیہ و یونانیہ) . From him we have the earliest distinction between smallpox and measles, and the understanding that smallpox occurs only once in a person's life. As a skilled chemist he recognized the toxicity of arsenic (arsenic oxide), but prescribed small doses of this compound in the treatment of many skin diseases and anemia.
He was a prolific author, who has left monumental treatises on numerous subjects. He has more than 200 outstanding scientific contributions to his credit, out of which about half deal with medicine and 21 concern alchemy. He also wrote on physics, mathematics, astronomy and optics, but these writings could not be preserved. A number of his books, including Jami-fi-al-Tib , Mansoori (کتاب المنصوری), al-Hawi(الحاوی) , Kitab al-Jadari wa al-Hasabah(کتاب الجزداری و حسابہ), al-Malooki (کتاب الملوکی) , Maqalah fi al- Hasat fi Kuli wa al-Mathana (مقالہ فی الکلی و المتحانہ), Kitab al-Qalb (کتاب القالب), Kitab al-Mafasil (کتاب المفاصیل), Kitab-al- 'Ilaj al-Ghoraba (کتاب العلاج الغربا), Bar al-Sa'ah (بارالصابا), and al-Taqseem wa al-Takhsir (التقسیم و التخسیر), have been published in various European languages. About 40 of his manuscripts are still extant in the museums and libraries of Iran, Paris, Britain, Rampur, and Bankipur. His contribution has greatly influenced the development of science, in general, and medicine, in particular.
In contrast to Jabir, who inclined toward numerical mysticism, al-Razi became practiced in experimental work. This is apparent from his two most influential works, Kitab al-Asrar (کتاب الاسرارThe Book of Secrets ), and Kitab sirr al-Asrar (کتاب سرالاسرار -The Book of the Secret of Secrets ). In these works he gave several recipes for the alleged transmutation of common metals into precious ones, and crystal or glass into precious stones. Perhaps al-Razi's main contribution to chemistry was his attempt to systematize laboratory practices, to which end he listed contemporary laboratory equipment and techniques used in chemical experiments. Another influential contribution to chemistry was his classification of all the chemical substances he knew, for this is the earliest attempt of which we are aware. Al-Razi divided these substances into four main groups: vegetable, animal, derivative, and mineral. The last group consisted of six subgroups:
(1) spirits (volatile substances, such as mercury, sulfur, and arsenic sulfide);
(2) metals (gold, silver, copper, tin, iron, lead, and "karesin," probably a bronze composed of copper, zinc, and nickel);
(3) stones (ores and minerals of iron, copper, zinc, but also glass);
(4) atraments (metallic sulfates and their derivatives);
(5) boraces (borax, but also sodium carbonate [confused with borax]); and
(6) salts (in which categorization sodium chloride appears under four different terms, other salts being sodium carbonate, potassium carbonate, and others).