I'm not actually sure if it's common or not. I think some people are more suseptible than others. I for one always use Sudafed 12 hour relief because I only have to remember to take it in the morning and I've got a dreadful memory for taking pills.
The trouble with some cold remedies and SS is that they may contain sugar. If they do you risk coming out of ketosis and therefore getting hunger which may cause you to eat something. However I'm of the opinion that with everything I have going on, I need the medication because I can't just stop what I'm doing and go to bed because I've got a cold.
I don't think Neurofen cold and flu is going to contain much in the way of sugar if it's pill form. Just check the packet.
I personally wouldn't suffer I'd take the medication, but that's just me.
If you were further into the diet than two weeks, the option would also be to have a hot berry or orange water flavour with some paracetamol crushed up into it. But that doesn't give you the decongestant. And you're not supposed to have the water flavours for the first two weeks now.