Dear Dr. Roach: Would it be risky to take a low dose (20 mg) of over-the-counter lithium orotate to treat cognitive decline? The recent study in the journal Nature recommended waiting for further studies to look at the benefits and risks of taking this compound. Given the promising findings in mice/rats and the anecdotal evidence of reversing memory loss, I’m inclined to roll the dice and give it a whirl. — J.H.R.

Answer: I can understand your enthusiasm. There have been several studies (some in mice, like the Nature study you mentioned, but others in humans) that have consistently shown a slowing of disease progression and a reduced risk of developing Alzheimer’s disease in people with mild cognitive impairment. Interestingly, places with low amounts of lithium in the drinking water have more dementia, leading some authors to speculate that “lithium deficiency” is a risk for dementia.

In the studies done on humans, doses were carefully adjusted to a blood level where side effects are less likely. Lithium used for bipolar disorder at doses that are roughly double the ones used for dementia prevention can cause serious changes to the regulation of salt by the kidney and brain, as well as thyroid abnormalities.

Since the metabolism of lithium is highly variable from person to person, the dose you are speculating about might not be enough to get your blood levels into the range that was used in the studies. But it might be high enough to put you at risk for serious side effects. I don’t know if your doctor would be willing to check your blood level, which is how the studies were done.

In my opinion, the literature does not yet support the use of low-dose lithium to prevent dementia.

Dear Dr. Roach: How often should adults get a tetanus booster? — A.T.B.

Answer: Adults who completed the primary series for tetanus and diphtheria should get a booster dose every 10 years. For a person who has a minor wound without contamination from dirt, they should only get the vaccine if they had it more than 10 years ago or if it is unknown whether they had the complete series.

A person with a tetanus-prone wound, or any wound that isn’t both minor and clean, should get a tetanus booster if they haven’t had one in five years, as long as they completed the primary series. People who are not known to have had the primary vaccine get both a tetanus booster and human tetanus immune globulin.

I can’t emphasize enough how important it is to be up-to-date with your tetanus vaccine. Tetanus is often called the “unforgivable” disease because it is much better to prevent than it is to treat. In a country with a working health care system, nobody should get tetanus.

In adults, tetanus is combined with diphtheria (“Td”) or with diphtheria and pertussis (“Tdap”).