Are OTC laxatives ok for a cat?

My cat was diagnosed megacolon about 7 months ago. 3 days in the ER, multiple enemas and thousands of dollars late, he was in the mend. He's been doing great since and has gained a lot of his weight back.

This week however, he's having a flair up. Mainly hard nuggets outside of the litter box. I gave him what I had left of his lactulose which was only about 2.5ml (or whatever the measurement on the syringe is lol) and gave him more miralax. I had been giving him miralax once a week.

His stool has definitely softened but he's still not going in the boxes (he has 2) and it looks like he's straining. I called my vet for a refill of the lactulose or cisapride and they refused. They told me to go back to the ER. Basically ensuring that I pay another several thousand dollars. I don't think that's necessary. He's still eating, drinking and playing. I think I caught it early enough that he just needs one big poop lol.

My question is is it safe to give him something that isn't prescription? I'm specifically looking at buying the BestLife4Pets - Cat Laxative. Is there another option anyone would suggest?