To avoid developing a vitamin deficiency, you should get the recommended amount of vitamins. But there is little evidence that taking more gives you a mental boost.

That is, there is no evidence that taking “megadoses” of vitamins either slows intellectual decline or prevents dementia.

The answer to this question depends entirely on the condition for which antibiotics have been prescribed.

Many bacterial infections that require antibiotic treatment are not contagious. And they may be cured with a short course of antibiotics. For example, uncomplicated infections of the urinary tract may be cured in as little as 1 to 3 days of treatment. Other infections take longer to cure. For example, a sinus infection may take 2 to 4 weeks in many cases to get better.

The most common contagious infections, including the common cold, involve viruses invading the upper respiratory tract. For these viral infections, antibiotics are not helpful to treat the infection. And they will not prevent the spread of the illness to others. “Strep throat” may be less contagious after a couple of days of antibiotics. But doctors recommend a full course to lower the risk of rheumatic fever.

Some sexually transmitted diseases (such as gonorrhea and chlamydia) may be cured with a short course of antibiotics. For example, a single injection treats gonorrhea. And a single large dose of an oral antibiotic can cure chlamydia. Similar treatments are given to sexual partners. To prevent the spread of these infections, people should not rely on antibiotics alone. They need to wait a few days to be sure that the drug was effective. And most importantly to practice safe sex.

For other contagious illnesses treated with antibiotics, it usually requires more than 3 days of treatment. This includes tuberculosis, other types of pneumonia, meningitis and bowel infections. Certainly, for these types of illnesses, the “3 day rule” is a myth. And relying on a short course of treatment is not recommended.