My iPhone battery is swelling is it still safe to use?

You need to have it replaced it as soon as possible, since it has reached the critical point of its life.

Get an estimate here (choosing your country):--> Choose Your Country or Region - Apple Support

Regards

Giulio

If it’s swelling you should immediately get it replaced by a apple authorised service provider.

This link should be helpful: iPhone Battery Repair & Replacement - Apple Support

In fact you should stop using it immediately. Swelling batteries are an explosion/fire hazard and could cause personal injury or worse.