Myself and my friend both started IV Doxy two months ago. Our herx varied in intensity. Hers was worse. We both have CNS lyme, and I also have moderate brain encephalitis. Anyhow, the herx for us both was pressure headaches all over, she had neck pain (I didn't), she had eye pain (I didn't), I had about 4 migraines the first month and still have constant headaches but after 6 weeks they aren't really so bad any more, I function fine. My head felt made of lead and 4 times its size for 6 weeks. I was in bed for 6 weeks, and also very dizzy.
I hate to tell you that it was the worst herx I ever had, but it was. However, I do believe my brain infection was also at the worst point it had ever been, so that makes sense. If you don't have Central Nevous System lyme you might not herx like this, I think the doxy if very good with brain and nerve infections. I have lyme, bartonella and babesia. The Doxy is for the lyme and bart, I take 2 malaron a day for the babesia and nystatin. I take Silphos to protect my liver and various vitamins when my stomach feels good enough. I tried sauna and it was a nightmare combo for me. I don't recommend you sauna on IV doxy, but I could have just had a weird unique reaction.
Anyhow, thankfully the worse is now over for both of us. Niether of us have been able to acheive the top theraputic dosage due to our herxes. I have gotten up to 100 mg twice a day and think I will slowly be able to hit 200 twice a day. The key for us both to surviving the herx was to cut back the dosage and go up slower, I even skipped two days here or there just to ease the pain. Other side effects I had were dry mouth and overall dryness. Brain fog. My mood was suppressed but not as crazy as it had been earlier so mood wasn't really the focus of the herx, it was headaches. I wish you the best of luck with the IV Doxy. It has made my life better after 2 months, but the first 6 weeks were tough.