Not true.
Drugs are often prescribed for "off-label" use. This is especially true for cancer patients. If the above were true, medical attention for an illicit drug overdose would never be covered.
Yes, drugs are prescribed for off-label use, and they key there is that they are prescribed by a doctor.
I'm not familiar with health insurance specifically and I'm sure it has its own specific general principles that set it aside from other types of insurance. Insurers are generally happy to cover a person engaging in a somewhat risky activity provided that someone with some official credentials has (by selling or supervising or otherwise approving) made the judgement that it is an acceptable amount of risk being undertaken. You can be covered for an injury you get skiing down a very steep run at a ski-resort, but not covered for an injury if you go skiing down a gentle hill near your house (for example). Your auto insurance might cover you for an accident the day before your license expires, but not the day after, even though your ability to drive safely hasn't actually changed. Your computer is still under warranty when someone at Circuit City breaks it, but not if you attempt to make certain repairs yourself (even if you previously worked at CC). And so on.
Strayling, difference companies and different policies will vary. You might be stuffed as Bob says, you might not. Call them, ask them, make your own record of the call, staple it to your insurance documents, and try to get them to follow up your call with written verification of what they've told you.