I would say no it is not harmful in the sense that the chemicals are not going to do damage to your body and you can't overdose or anything like that. However, if you ever decide to try it, make sure to do it in a comfortable environment (a party would not be a good place) and make sure that someone with you is not going to be taking any Salvia (so have a sober person to watch out for people taking it). People have different reactions to it and so the danger can come in the form of unknowingly placing yourself in dangerous situations because when you're high on Salvia, things become very strange and distorted and there is definitely no sense of realism or reality. With that in mind, it would be very easy to make a dumb decision like running out into traffic or thinking that you can fly and trying to jump off a building. I didn't personally ever get any notions that I would be able to fly off the top of a building but for some people I would not doubt that they might think that and possibly try it. Other than that, i have nothing against Salvia as long as it is taken responsibly and carefully.
And the "high" effect only lasts at tops for 5 minutes. After the 5 minutes you come down from the high extremely fast and after another 5 minutes the high is completely gone.