Can hypnotherapy be effective in treating ptsd?

Yes. Emdr is another effective accepted method for ptsd treatment. It may take several sessions but for many people it is the only thing that cures it for them.
Yes. Hypnotherapy has been very effective for certain individuals regarding different medical concerns or illnesses. It would be wise to thoroughly research specific hypnotherapists prior to choosing one to help. Also, as with all complimentary or alternative medical treatments, results can vary widely so what works very well for one person may not be effective for another.