For how long (and how often) should i take doxycycline 100mg for a groin lump (im thinking its probably an ingrown hair or a boil beginning to develop)? thanks?

Unless there is a sign of infection, there would be no indication for taking doxycycline at all. If there may be infection present, then you should take the full course of doxycycline or whichever antibiotic is prescribed. Do not take a partial course. See your doctor if there is any question of infection. Good luck.

You should not be taking doxycycline for this sort of problem unless it was prescribed by a doctor. Probably no antibiotic is needed, especially this one. See a doctor if the problem develops into a large, tender lump or if you develop fever.

Groin lump should be examined and treated by your doc. pl. see her/him. PL do so. Never use antibiotics to treat a self diagnosis.