Managing a cat colony requires the understanding and support of at least one veterinarian. I am extremely fortunate that I can rely on two talented and brave doctors, both of whom are NOT afraid of dealing with feral cats, and charge greatly reduced fees for treating this population. From time-to-time ferals do become sick or injured and need to be re-trapped and taken in for treatment. Fortunately, having a vet who will provide you with an antibiotic such as clindamycin is helpful because the cat can often be treated at the colony without the stress of being trapped and caged at a clinic.
Samson's first injury was a deep bone injury, so I gave him 150mg of clindamycin once a day for two weeks. Medicating him with 75mg twice a day, the way I might for my house cats, was not a possibility, so he got the full daily dose at one sitting. For most caregivers, getting a feral to eat medicated food once a day is about all one can hope for. I found the best method was to mix the content of the capsules in two to three tablespoons of his favorite food (Fancy Feast Ocean Fish Feast) in a small paper dish. I also placed a few tablespoons of the food on dishes for the other cats, so that all would have their own and leave Samson alone. One of the hardest parts of medicating is making certain the cat in question is the only one that eats the dosed food. I have been told by experienced caregivers not to worry if the cat does not ingest every morsel of medicated food. While it is best for the cat to receive the full protocol of medication, my cat whisperer friends also reassure me that four or five--and not always consecutive--doses over several days is better than nothing. I saw this play out with Thomas Moore who had an abscess on his back leg. I was not able to medicate him every day, as he did not always show up at the feeding station. However, four doses of 150mg of clindamycin spread out over eight days seemed to do the trick and he recovered quite well.
Let me add here, however, that had Thomas or Samson not shown improvement in a reasonable amount of time--say, two weeks, I would have tried to retrap. Re-trapping is a topic unto itself. If you have a cat that refuses to go back into a traditional trap I have one suggestion: find someone who is familiar with using a "drop-trap"