What are the main reasons that makes translating medical documents to Arabic is very difficult?

Because studying medicine in English so most terms used in English

As per my experience, translation of medical documents from english to arabic is not that difficult. One main condition is necessary is carrying a large scale research on all familiar words that maybe translated into arabic like parts of the human body, diseases, medications...

Because most of medical words are Latin and difficult to pronounce it , translate it and keep it in mind. Also English language that's used in medicine is somehow difficult.  In addition,  spontaneous use of languages or inaccuracy. 

Yes Shawqi and as Abonasser has stated above, the apparent reason is that Latin language is the source of almost all medical terms. In most cases no equivalent in Arabic and even difficult to find sometimes in English.

You must have a knowldge base of Pharmacology or you have to search and to read about every word, Definition and Theory.

According to my experience in translation, I thing that the lack of Arabic tremionology makes some translation works from English into Arabic difficult

Medical translation is difficult because a translator would deal with terminologies and expressions originally coined and used in English language. Unfortunately the Arabs could not cope with that, I mean finding equivalents for these terminology in Arabic language.

Anyhow, now it is not that so difficult to translate medical documents; for example, WHO has created a comprehensive dictionary for medical terms and expression, especially complicated names and expressions relating to the field of medicine.

Efforts are being made every day to facilitate this, specially for medics who almost study in English language, and faced with difficulties when they deal with Arabic texts relating to the subject! 

 because most of it at latin language so its 

Impossible to use 

 Literal translation

Arabic is not a language of science. Translators will more easily find the Western term as compared to when trying to find the equivalent in Arabic. Moreover, translators usually focus more on their foreign language abilities comparing to their native Arabic skills, especially if the translator comes from a scientific background. We can see this clearly in newspapers and magazines.