Can physicians countersue for frivolous lawsuits?
Edit: I suppose a better question is do they ever counter sue?
In theory yes, in practice no. First, you have to realize that what
you consider a frivolous lawsuit probably isn't. A frivolous lawsuit is one that has no basis, and cannot survive a summary judgment. It's not merely one where the defendant prevails because his evidence is stronger. Thus if a plaintiff can make a claim that is supported by an expert witness saying that X didn't follow the standard of care, it's not frivolous by definition, even if it's deemed over the course of the suit that in fact it wasn't the standard of care, or that the actions didn't occur quite the way the plaintiff painted. Those kind of cases aren't frivolous, and a courtroom is actually the appropriate place to sort things out under the American system.
In general a judge will dismiss a truly frivolous suit outright, and often will impose heavy financial sanctions on an attorney who brings a frivolous suit, and thus most of these groundless suits get weeded out in the attorney's office and never see the light of day. Physicians don't appreciate how many stupid waste of time lawsuits lawyers filter out before bringing the ones they bring, but it's pretty substantial. For every obnoxious TV ad you see on TV, there are dozens of folks who call up the firm but if they get one good case they would consider filing out of a given airing of the commercial that's a lot. There was a Harvard study a couple of years ago that analyzed "frivolous lawsuits" in medmal and determined that they were actually an extreme rarity, not something most physicians actually will ever experience. Most physicians will be sued over the course of their career, and those suits will not be frivolous, although in most cases the physician will still prevail.
As for countersuing, you have to realize that in law for every action there is going to be a reaction. If you make claims against the plaintiff, he gets to respond. So a relatively benign suit that you would otherwise win may end up being an awful quagmire for the physician if you put the plaintiff on the defensive. Suddenly all those things that bothered the patient about his doctor that he wasn't going to bring up come out, and you never know what's going to stick with a jury. You take it from a very clean, "was the standard of care met" kind of case and muddy it up with a "who is the bad guy" kind of case. And the plaintiff already has some modicum of sympathy on his side because he or a loved one has been damaged. So no, you want it to be a passion-less battle of experts, staying on point, with no finger-pointing back and forth. Not to mention that the typical patient with hefty hospital bills isn't going to have any money you could win, and isn't insured the same way the doctor is, so even if you did prevail so what -- you are still lucky to see a cent, you still have the lawsuit on your permanent record, and you will have spent more money on litigation costs. It's a lose lose situation.