http://www.mayoclinic.org/drugs-supplements/vitamin-b6/safety/hrb-20058788
Vitamin B6 may cause abnormal heart rhythms, acne, allergic reactions, breast enlargement or soreness, changes in folic acid levels, decreased muscle tone, drowsiness or sedation, feeling of a lump in the throat, feeling of tingling on the skin, headache, heartburn, loss of appetite, nausea, rash, recurrence of ulcerative colitis (an inflammatory bowel disorder), stomach discomfort or pain, sun sensitivity, vomiting, and worsened asthma.
Side effects to some ingredients of high-dose pyridoxine hydrochloride (which is injected into the vein) are possible.
Vitamin B6 may cause low blood pressure. Caution is advised in people with blood pressure disorders or in those taking drugs or herbs and supplements that lower blood pressure.
Vitamin B6 may affect blood sugar levels. Caution is advised in people with diabetes or low blood sugar, and in those taking drugs, herbs, or supplements that affect blood sugar. Blood sugar levels may need to be monitored by a qualified healthcare professional, including a pharmacist, and medication adjustments may be necessary.
Vitamin B6 may increase the risk of bleeding. Caution is advised in people with bleeding disorders or taking drugs that may increase the risk of bleeding. Dosing adjustments may be necessary.
Use cautiously in people who have heart conditions or stomach and intestine conditions.
Use cautiously in people taking agents for Parkinson's disease, as they may interact with vitamin B6.