Joran Geran is nearly the most experienced Medic in the Chapter. He is the Master of the Apothecarion, but several of his junior staff are for more skilled in the healing arts.
Geran's success is due mainly to the assistance granted to him by the tiny people from one of the moons of Chaktaaron. Years ago, he made a pact with them. No-one is entirely sure what he gave up (probably his peanut butter and jam sandwich rations) but ever since the little creatures have followed him about in droves, acting as servants and helpers and advisors and generally lending a helpy hand.
In battle they are invaluable, rendering medical assistance to just about anyone in sight, and Geran has begun to carry multiple medipacks because the little guys keep running out of chemicals.
For his own part, Geran carries a Tyranid Deathspitter (a gift from the Hive Tyrant T’th’tok’ren’ko’shon-ta when he visited; Geran was the only Marine who would go near the foul thing) backed up with a power sword and a needle pistol, for dispensing hallucinogens and other psychotropic drugs at a distance. If you can't enjoy your job, what can you do?