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Q. I was troubled with recurrent canker sores for a while. The only thing that had changed was my toothpaste. I wanted whiter teeth, so I was using whitening toothpastes.

I got canker sores when there was sodium lauryl sulfate (SLS) in the toothpaste. As soon as I switched to toothpaste without SLS, the mouth sores went away.

A. Many readers report that if they avoid toothpaste with SLS, mouth ulcers are less troublesome. Nevertheless, a randomized controlled trial of SLS-free toothpaste did not result in a change in outbreaks. It did cut healing time and reduce pain (Oral Diseases, October 2012).

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Q. Taking Avodart triggered the onset of both erectile dysfunction and reduced ejaculate for me. I took it for two years; my prostate did not shrink, but my penis did.

These sexual side effects linger. Five years later, I still have ED and low ejaculate. The symptom of frequent nightly urination, however, has ceased to be a problem. Will the sexual side effects ever go away?

A. Medications that shrink the prostate include dutasteride (Avodart) and finasteride (Proscar). These drugs are prescribed to treat symptoms of benign prostate enlargement. Lower-dose finasteride (Propecia) also is prescribed for male pattern baldness.

A study published in the Journal of Sexual Medicine (November 2012) reported persistent sexual side effects in men who took finasteride. These included reduced sexual arousal, lower ejaculate volume, erection difficulties and inability to reach orgasm. The study subjects also complained of smaller penis size and reduced sensation. It is not clear how long these complications may last. Dutasteride is likely to have similar side effects (Urology, March 2014).