Can I be denied a job in the medical field due to the fact that I am a certified medical marijuana user?

Yes, you can be denied a job (or fired from a job) under the following three circumstances:
1) You bring, or state you will bring, the marijuana to work. The problem you face is that even if medical marijuana is legal in your state for your use, it is still an illegal drug (a CDS or controlled dangerous substance) under federal law. Employers cannot be forced to allow violations of federal law on their property/at their worksites.
2) You are under any influence of marijuana at work: you can be denied a job or fired if you are intoxicated by any substance (including ones completely legal, like alcohol) in the workplace.
3) The workplace is itself subject to regulations that require it to bar anyone using certain substances from it or from the job you want, including in this case marijuana (e.g. many commercial driving jobs do not allow drivers who are under certain medications), because the employer cannot be made to violate regulations governing it.
Otherwise, if you are merely a registered or certified user and do not bring it to work, are not under the influence at work, and are not in a workplace or position where you cannot have the job if using marijuana, they cannot deny you employment. If you are being denied employment when there is not good cause as per the above, contact the state department of labor (who may either help you themselves or may refer you to another agency which is better suited to assist).