Hi there, thanks for the query. You are right, since back pain could also lead to sleepiness during the day. In fact any active pain in the body can inhibit body and brain to fall into deep sleep and so can bring daytime sleepiness. It is good that your pain be managed, and it is important that it managed non-medically or with a medication from NSAID family, like ibuprofen (if you do not have any peptic ulcer disease or kidney problem because ibuprofen could be harmful then). Codeine and the similar drugs are not appropriate in this case, because they suppress the respiration at night and bring more sleepiness during the day. You can also ask your doctor to prescribe a medication other than zyrtec for your allergy (like loratadine) to have less sleepiness. If you use them in the afternoon, it would be better, because antihistamines change architecture of your sleep as well. I wish you the best sleep and health.