Oyster shell calcium for osteoporosis 500mg 3 x a day. is there any side effects? and what is the likelihood of having anaphylaxis from it?

Exceedingly small: on anaphyllaxis, doubtful since mostly they use very old oysters dredged up that don't have living protein in them. but, then, the calcium isn't all that different from calcium in any other mineral form. Might be more effective if magnesium is also given with the calcium. Without an actual examination, I cannot give specific medical advice on your particular health,

Unlikley: Allergic reactions to calcium are very rare. However I do not know your underlying allergy history. You seem very young to have osteoporosis so you need a through workup as to the cause. Calcium alone for treatment of osteoporosis is not very effective. Calcium can cause constipation. Calcium supplementation alone is unlikely to prevent osteoporosis unless you are nutritionally deficiency