Q: I am a female in my mid-60s and recently have been diagnosed with a cyst on my left kidney. I do not have diabetes nor a heart condition, never smoked nor used drugs, don’t drink and take over-the-counter or prescription medicines only when absolutely necessary, which is rare. I also don’t like, thus never eat, spicy foods, nor do I drink coffee/tea/caffeine. I have only an occasional candy bar.

I know that kidneys filter out bacteria but am still wondering how could a cyst, or any renal ailment, be bestowed on such “clean” kidneys? Can you please also elaborate on kidney cysts?

A: A cyst is just a fluid-filled sac, and they are common in the kidneys, especially as people get older. They are more common in men: Among adults aged 50 to 70, 15 percent of men and 7 percent of women have had a kidney cyst.

It does not seem that diet or medication predispose someone to developing kidney cysts. They may arise from diverticula (outpouchings) in the kidney’s collecting tubule. Obstruction of the tubule, such as by a stone, may predispose to them, too, but it is certainly not necessary for a stone to develop a kidney cyst.

Most simple cysts do not require treatment. After it is clear that the cyst is not cancerous (rare), it can be left alone, or sometimes may require a follow-up scan to be sure the cyst is stable.

Incidentally, the kidneys shouldn’t have to filter out bacteria. The kidneys filter small- and medium-sized waste products from the blood and help regulate salt and water balance. Bacteria shouldn’t be in the blood for more than a few minutes (bacteria sometimes come through the body’s defenses, especially in the gastrointestinal tract, from the mouth all the way to the end): There’s no reason you should have bacteria in your blood.

Q: I find the warnings about limits on taking acetaminophen (Tylenol) to avoid liver damage very confusing. Regular and extra-strength versions of acetaminophen warn not to take more than 4,000 mg in 24 hours. Other formulations say 3,000 or even 2,000 per day. If I take one Excedrin PM with 250 mg of acetaminophen at night, what are the limits on other acetaminophen products the next day? I sure don’t want to risk liver damage, but acetaminophen doesn’t cause the digestive problems of NSAIDs. What if you are prescribed Norco?

A: You are right that different over-the-counter products containing acetaminophen have different limits printed on the label. For healthy adults, the limit is 4,000 mg. For people who use alcohol excessively or those with mild liver disease, even that amount is still safe, according to studies, even though many physicians (including myself) would recommend a somewhat more cautious threshold, such as 3,000 mg daily.

What’s very important is to recognize that many over-the-counter medicines have acetaminophen in them. Also, prescription medications, such as Norco (hydrocodone with acetaminophen, also sold as Vicodin or Lorcet, among others) and Tylenol No. 3 with codeine have acetaminophen as well. People on multiple medicines need to look carefully at all sources to total up their acetaminophen dose for the day to keep their total below the recommended dose.

Dr. Keith Roach’s column runs in Wednesday’s Extra.