Chief medical officer The chief medical officer or ship ' s surgeon was a senior staff - level position , typically held by a lieutenant commander or commander who was a doctor of medicine with surgical skills . A nurse could also hold this position if no qualified doctors were available . ( ) Chief medical officers were also referred to as the " ship ' s surgeon " in the 23rd and 24th centuries . ( ; ) Some noted chief medical officers in Starfleet history were : The Starfleet of the alternate reality established in 2233 by the temporal incursion of the Romulan Nero had one noteworthy chief medical officer : Typical ranks . Chief medical officers have held a variety of ranks . They could hold any rank in the lieutenant grades , but were quite often seen as lieutenant commander s . Chief medical officers were eligible to take the Bridge Officer ' s Test for promotion to full commander . At this point , they were considered capable of assuming command of a starship . ( ) In light of that , their primary duties remained the crew ' s physical well - being , and several chief medical officers have been civilians with no clear role in the ship ' s chain of command outside their medical duties . ( VOY ; ENT ) Specific duties . CMOs aboard a starship or starbase were directly in charge of the sickbay or infirmary , commanding medical department personnel and activities . The CMOs ' primary duties were the oversight and maintenance of the overall health and physical fitness of crew and passengers . Furthermore as standard medical procedure , the CMO had full medical authority over any patient officially admitted into their care in their facility , including superior officers , until they were formally released from care . Officers likely to be assigned as the CMO over long - duration missions of exploration had scientific training in disciplines outside of medicine , allowing them to take on major , extended research projects , or to better participate in a wider variety of encounters . ( ; ) Medical authority . In addition to the normal duties of an officer , Starfleet Order 104 Section C gave CMOs the power to relieve an officer or crewman , including a superior one , of their duties if in the CMO ' s professional judgment , the officer in question was medically unfit . The CMO in question would need to officially certify and provide evidence of unfitness at a subsequent board of inquiry . ( ; ) Typically , the CMO was expected to have the support of a senior command officer or detailed medical documentation to justify this course of action . ( ; ; ) The chief medical officer was able to impose upon crewmembers medical physicals if the mission lasted more than two weeks . ( ) The CMO also seemed to have the authority to order officers to do things that would be beneficent to their health , like sleeping . ( ) In the 23rd century , chief medical officers certified the health of research personnel on alien planet s on a yearly interval . ( ) " In an alternate timeline on stardate 51425 , The Doctor cited Starfleet Medical Regulation 121 , Section A in an attempt to relieve Captain Kathryn Janeway of her command . She refused his order , stating that he had no way to actually " implement this protocol " . He countered , by saying that he would note the incident in his official log , and that it could lead to her general court martial . " ( )