Metoprolol side effects usually include dry mouth, constipation, heartburn, vomiting, diarrhea, decreased sex drive, impotence, or difficulty having an orgasm, headache, drowsiness, tired feeling, sleep
problems or anxiety and
nervousness. (ref: http://www.drugs.com/metoprolol.html) Nipple pain and tenderness have been documented but very rarely. Other factors that may have an effect may include hormone changes. Pain reliever medications and warm compress may help provide relief.