Is it possible to genetically engineer a tapeworm that is beneficial for weight loss, without the usual negative symptoms?

What traits would you engineer for? Just Google “tapeworm diet” and lots of info comes up with delightful details. :-)

It’s been done. There are no less negative impacts than yo-yo fad diets.

I think I’d rather have my stomach stapled.

It’s pretty gross to have a worm living in your body. Even worse, they tend to multiply. If you could prevent the multiplication, it would be less gross. Luckily there is a safe therapeutic dose for Ivermectin for that.