Oct 31, 2018. Along with its needed effects, doxycycline may cause some unwanted effects. armpit, or groin; tightness in the chest; unusual tiredness or weakness. Some side effects of doxycycline may occur that usually do not need. NHS medicines information on doxycycline - what it's used for, side effects, dosage and who can take it. Do not drink alcohol while taking doxycycline.