Cambridge don't do a laxative. They do a laxative preventer, but you wouldn't use that if you had a problem.
You can use senna if you are constipated. Don't use it regularly though. Better to use a preventer daily ie Fibre 89, fibresure or psyllium husks. They are safe are safe to use regularly
Are you positive about this, because it should be the other way around. Ducolax is much stronger and not recommended on Cambridge. Senna is okay though
I tried Andrews Salts which worked for me and I managed to lose 8lbs the first week and 8lbs the second week so I guess it isn't kicking me out of ketosis.