Right now, I’m suffering from a pretty bad cold - and have been taking over the counter antihistamines - Sudafed’s “grocery store pharmacy in house brand equivalent” anyway - and have noticed that since I’ve started taking it, I’ve been having more bad dreams at night.
Is it possible the two could be related? Could the increase in bad dreams be a side effect of these pills, or simply a coincidence since I feel so allover crummy?
Actually, if you are having nightmares it is most likely do to a fever you may have and not the antihistamines you are taking. Fevers are often accompanied by mightmares, IANAD, but it may have something to do with elevated core temperature. I know when I have a fever I often get horrible nightmares. Try accompanying advil or some type of Ibuprofin before bed, it may help.
You experience nightmares because of the ill humours that are in your home. I prescibe a healthy blood-letting, followed by a dose of laudenaum.
Or,antihistamines can cause nightmares. In children, at least.
Thank you both. 
Also, cold remedies can cause constipation…and the inner discomfort can find expression in dreams.
Try Nyquil for some really bad nightmares! I doubt that antihistimines work on colds. I think they are for allergies/hayfever.
You’re lucky to just have bad dreams. Ordinary Sudafed or Actifed don’t make me drowsy. But if I do go to sleep under their influence, I get night terrors complete with visual hallucinations. brr.
Non-drowsy Sudafed doesn’t have these effects on me. Yay Pseudoephedrine!!
Now, I’m more bothered by feeling I always have to list Actifed/Sudafed as an “allergy” to a medicine on whatever new doctor or dentist I have to go to.
AmbushBug
Since they make me nervous, I find I have bad dreams after taking Sudafed. All night long I’ll dream of losing things, being late, whatever puts me on edge during waking hours, blech.