Decongestants like Sudafed can cause dehydration, interfere with REM sleep due to elevated epinephrine, and lead to lightheadedness or jitteriness.

“They do have a couple of effects that are not so great for the course of treating the underlying cause, which are first of all they can cause dehydration. So you have to make sure that you're hydrating well, both fluids and electrolytes, and they also can interfere with sleep because as I've talked about in the episodes on sleep, one of the hallmarks of deep sleep and in particular REM sleep is that epinephrin, adrenaline levels are low. This is what allows you to have intense, often very emotionally-laden dreams during REM sleep and not act those out. And low adrenaline, epinephrin during REM sleep is basically a signature, a neurochemical signature of the REM sleep state, which is so vital for emotional and physical repair and so forth. So, the fact that they can inhibit sleep, the fact that can cause dehydration, the fact that they can make people feel kind of lightheaded and jittery makes them not terrific choices for a number of people.”