Abby: ‘Narrow therapeutic use’ standards under review

Keith Roach
To Your Health

Dear Dr. Roach: You wrote recently that thyroid medication is regulated to within 5 percent of the stated amount. Are there other drugs that are similarly regulated? Are generic manufacturers held to a different standard than the manufacturers of brand-name drugs?

A.L.S., M.D.

Dear A.L.S.: The Food and Drug Administration continues to work on new standards for drugs that are considered “narrow therapeutic index,” meaning the dosages have to be precise, due to the fact that these drugs can be more than usually toxic at a high dose or that the consequences of too low a dose also are more dangerous than other drugs. These drugs include anti-epilepsy drugs, some other psychiatric medications, drugs to prevent rejection of transplanted organs, as well as thyroid medication and digoxin. For thyroid medication, all manufacturers, whether brand-name or generic, are required to meet the same standards.

I have attached a list of the FDA policies and publicly available internal documents, as well as some general and scientific information about generic versus brand-name drugs, on my Facebook page at

Dear Dr. Roach: I have peripheral neuropathy due to a pinched nerve. I get some relief from it, but it still hurts and keeps me from sleeping. My doctor recommended a Tylenol PM, and it helps me sleep well for six or seven hours most nights. But some nights I wake up hurting and have trouble getting back to sleep. Then I can take two regular Tylenol and, most nights, get back to sleep after about 30 minutes. My concern is the long-term use of the Tylenol PM.


Dear W.H.: Tylenol PM contains acetaminophen (the pain reliever in plain Tylenol) and diphenhydramine (an antihistamine with sedating qualities).

Excess amounts of acetaminophen can be dangerously toxic to the liver. This is especially important for people who consume alcohol regularly or have another kind of liver disease. Healthy adults should take no more than 4,000 mg per day (that’s eight extra-strength tablets, but since other medications also may contain acetaminophen, it’s the total amount that needs to be considered). People with mild liver problems should not exceed 3,000 mg per day, or should discuss with their doctor.

Diphenhydramine potentially can cause a large number of problems, but most people tolerate it reasonably well. It is not my first choice for a sleep aid, as it increases risk for falls and automobile accidents. Diphenhydramine is associated with an increased risk of developing dementia; however, it’s not yet clear whether the diphenhydramine really causes the dementia.

There are no risk-free medications, and peripheral neuropathy can be a painful, even debilitating condition. It sounds like you are getting pretty good results. The risk from the Tylenol component is small; the risk from the diphenhydramine component is unknown but also likely small. One option I would consider is trying regular Tylenol (not the PM) at bedtime. It may be that it’s the extra pain relief that is helping you sleep, and you may not need the diphenhydramine at all. Finally, a recent study showed that changing to a plant-based diet improved symptoms of diabetic neuropathy: It may benefit you.

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