Americans like simple answers. Is something healthy or harmful? We want medicine to be beneficial and not cause side effects.

Unfortunately, what works well for one person could be poison for someone else. There might be no good way to tell in advance.

The most frequently prescribed pain medicine in America is gabapentin (Neurontin). Roughly 10 million people receive a prescription for this drug annually.

Some get it after a surgical procedure for a few days. Others take it regularly to ease nerve or back pain.

Some people find it highly effective. One person states, “My doctor prescribed low-dose gabapentin off-label at bedtime to ease my nighttime joint aches. This medicine has allowed me to stop tossing and turning and get a good night’s sleep.”

Another adds that gabapentin stopped leg cramps: “What a blessing to be able to sleep the night through.”

Yet a third describes a partner’s trigeminal neuralgia head pain as a condition that could lead to suicide: “When he begins to feel preliminary electric sparks in his head, he takes gabapentin, and it controls the problem.”

Other readers have had terrible experiences with gabapentin. One reports: “My wife was prescribed a low dose of gabapentin for post-surgical nerve pain. It worked well for the pain. But after a couple of days, she developed significant side effects: blurred vision, memory loss, apparently some hallucinations, odd eye movements and loss of muscle control.”

Another man reported that his wife had hallucinations while she was taking gabapentin. We also received this tragic story: “My husband was prescribed gabapentin for back pain. While taking it, he became more depressed and isolated himself. We did not realize the link with gabapentin. As a result, his suicidal thoughts became actions.”

According to Dr. John, commenting on our website (www.PeoplesPharmacy.com), “Gabapentin is well-tolerated by about half of the patients for a wide variety of problems. The other half of people shouldn’t take it.

“I have seen patients get their brains organized for the first time and help their families. My life would be severely limited without some gabapentin because of disc disease in my neck after a car wreck.”

We don’t know if Dr. John’s estimate is accurate. His general point, though, that some people get benefits while others are grievously harmed, is exactly what these stories illustrate.

Gabapentin is not the only medication with a “split personality.”

The most-prescribed blood pressure medication in America is lisinopril, taken by more than 20 million people. It is known as an ACE inhibitor.

While many people who take it find that it works well with few problems, some people experience serious complications. Hundreds of people have written to us about an unbearable lisinopril cough. It does not respond to cough medicine and can be incredibly disruptive.

Even more serious is angioedema.

This potentially life-threatening reaction can affect the face, lips, tongue, throat or intestines.

Rapid swelling can interfere with breathing or digestive function. Abdominal angioedema can be hard to diagnose.

The bottom line is that drugs are neither good nor bad. Some people may benefit while others can be harmed. Predicting such outcomes in advance is challenging.

In their column, Joe and Teresa Graedon answer letters from readers. Write to them in care of King Features, 300 W. 57th Street, 41st Floor, New York, NY 10019, or email them via their website: www.PeoplesPharmacy.com. Their newest book is “Top Screwups Doctors Make and How to Avoid Them.”