Does it hurt?
It shouldn't. The only pain you might feel could be emotional pain if you're dealing with past trauma. It's not uncommon to cry a little, because you're releasing emotions and relaxing.
What are the risks?
Hypnosis is generally considered safe, if performed by a trained professional. There aren't many risk factors to hypnotherapy. It's rare, but you might headache, dizziness, nausea, anxiety or creation of false memories.
One risk factor is that it might not work. It's best to research hypnotherapists and hypnotists before using their services.
What's the follow-up?
The number of times people go back varies and is based on the severity of the case and the individual. For example, people going for weight loss who need to lose a significant number of pounds might go back five times. Others trying to quit smoking might go back fewer times. Sometimes after a year or so, people might want to go back for another session to refresh their minds.
Source: Charles Bender, hypnotist, chief financial officer and programs director at Chrysalis Institute in Moore; National Center for Complementary and Alternative Medicine; The Mayo Clinic.