<s>[INST] I had been prescribed erythromycin 250mg two tabs four times a day.it had side effects on me of loose bowls very tired generally feeling awful.this had been changed to doxycycline 100mg one per day. Could you explain the difference of the antibiotics. Thank you [/INST] Dear Friend. Welcome to Chat Doctor. I am Chat Doctor. Both the antibiotics belongs to different class. Both have the own indications. I would like to know that for what condition, you were prescribed these medications as there are a numerous use of these antibiotics. We use them for a lot many infectious conditions ranging from Sore throat to STDsMostly abdominal upset is seen with doxycycline. This is my best advise for you based on available details. Ask me if you have any further query. </s>