Have you ever heard a Witness argue that blood transfusions are often unhealthy anyway? I have... very often. It's something that they mention nearly every time to make you think... "Oh, after all, it's not like Jehovah's Witnesses are fanatics because they deny certain medical treatments purely for the sake of the Bible. They do have medical reasons as well!"
But here's the thing: this whole argument is a red herring, a bloody boogeyman that's supposed to make you feel that there's something wrong with blood transfusions from the medical point of view. Regardless of whether that's the case, it's an irrelevant argument. Here's why.
Let's assume that a Jehovah's Witness named Mark is experiencing hypovolemic shock caused by blood loss due to blunt trauma. Hypovolemic shock takes place if you lose around 20% of your blood. This is a medical emergency that necessarily leads to death if untreated. When in this condition, it's possible to be treated by using only a volume expander, but here's the thing: the more blood you lose, the less effective it is, and moreover, this argument that blood transfusions are sometimes harmful no longer applies in this scenario because blood transfusions are actually the best way to prevent death from hypovolemic shock. They are by no means harmful in this case.
So... would Mark be allowed to have a blood transfusion then? Or let's go even further; let's assume that his doctors tried normal saline (a volume expander), but the technique failed, and his only alternative is a blood transfusion. Is he allowed to accept it then, even if it's the only way to save his life?
No. And we all know that that's the answer. And why is it? Is it because "blood transfusions are sometimes harmful"? Obviously not. The reason is that the Bible says you cannot drink blood, and that therefore, you shouldn't accept blood transfusions either, even if death is absolutely certain without it.
That's why I never argue about whether blood transfusions are safe or not. Statistically, they're more likely to be safe than not, but it's not like Jehovah's Witnesses care. Even if it is absolutely certain that you'll die without it, they still will not allow you to accept it. That's why it's a red herring. The actual reason—the only important reason—is that this is how they interpret the Bible.
On a further note, have you noticed that this is often something that cults do? Whenever they create a rule that is non-conventional and obviously harmful, such as shunning or refusing blood transfusions, they always make up excuses to make it look as if they were doing these things for other reasons than the Bible. Just so that they don't look like fanatics. I mentioned shunning specifically. Have you ever encountered this argument? Jehovah's Witnesses shun disfellowshipped and disassociated people to ensure the congregation remains clean and is not compromised by sins of former Witnesses. One time, a Witness compared shunning to what people do when their friend steals from them. If someone steals something from you, you're going to shun them (after calling the police.) That's because they did something greatly immoral, "sinful." That's a red herring as well. Every single former Witness I have ever talked to left the religion because they no longer believed it was true, not because they wanted to lead a sinful life. I can conceive of examples where someone may leave the religion to lead a sinful life, but that's not what happens in the vast majority of cases. It's just another boogeyman that's supposed to scare active Witnesses. "Oh, don't associate with him! He's disfellowshipped, so he's leading a sinful life!"