I've had synto twice, but my OB had previously said he's reluctant to use it more than once, as there is an increased risk of bleeding, so if he uses it in subsequent pregnancies he said he doesn't leave the room.

I only needed it towards the very end of DD's labour. She was prem, and we both had high heart rates and my cervix was doing funky dilating things LOL (hows that for technical terms!).

I asked the same question here after my OB had said he doesn't like to use it in subsequent labours and lots of people replied saying they'd been induced more than once with it, so *shrug*.

As for the heart thing - no idea. But the side effects listed on the synto are quite numerous, so I wouldn't be surprised!