DEAR DR. ROACH: My question is about methocarbamol (Robaxin). My relative has been taking this for about two years for back pain. He then developed a sudden severe allergic reaction and went into anaphylactic shock. He fought it all night, not knowing what was happening, and by morning, he had to go to the hospital. Could this be due to methocarbamol? If so, can this happen again?

He had no known allergies, and this had never happened before. The only other thing was maybe burning a lemon-scented candle in his bedroom, but he has burned candles before in his bedroom. He is 27 years old and in good health.

— B.G.

ANSWER: Anaphylaxis is a severe allergic reaction to a foreign protein or carbohydrate (called an “allergen”). It is caused by a release of inflammatory chemicals from immune cells. It can be antibody-mediated (more common) or occur without antibodies (less common).

These chemicals can cause swelling of the airway (and death through asphyxiation). They can also affect the heart and blood vessels, causing shock and circulatory collapse, which can also lead to death. The onset of symptoms is typically minutes to hours after exposure to the allergen.

In 95% of cases, the symptoms disappear, but there are cases of “biphasic anaphylaxis” where symptoms reappear usually within 12 hours. There are unusual cases of very delayed anaphylaxis, such as with alpha-gal syndrome — a meat allergy that is acquired after tick exposure.

Medications are one of the most common causes of anaphylaxis, and I did find case reports of anaphylaxis with methocarbamol in a person who had been taking it previously. I found another case report of a person with prolonged anaphylaxis after methocarbamol. It is uncommon but not impossible to develop anaphylaxis to a medicine that you have been taking for years.

If your relative is not taking any other medications and had no unusual foods on that day, I strongly suspect that methocarbamol was the cause and recommend never taking it again as it can absolutely happen again.

Candle smoke allergy is documented, and a variety of symptoms can be caused by candle smoke in sensitive individuals. I found a source saying that anaphylaxis from inhaling candle smoke is “rare” but could not find a well-documented case.

Finally, methocarbamol is a muscle relaxant and is not always a good choice for long-term treatment. My experience is that in many people, the beneficial effects wear off as the body gets used to the medicine, which can happen within a week.

ToYourGoodHealth@med.cornell.edu.