Sugar-free ketchup has artificial sweeteners. I generally don’t trust anything that’s labelled sugar free because of that.

If you’re trying to lose weight, skip the ketchup if you can. Replace it with something not as sugary. If you’re using it to dip your fries, well, the fries are going to get you more than the ketchup but even changing out the ketchup for malt vinegar (a la Five Guys) will help.