The most important reason is to detect rare reactions.

There are a few drugs today that can cause this kind of rare reaction in some people.

Mrs Barnett, 35, suffered acute liver failure after a rare reaction to a drug.

Although the risk of problems from vaccines is already extremely low, these agencies watch for any reports of rare or unexpected reactions.

In the middle of his procedure her husband had a rare reaction to the anesthesia.

Anaphylaxis is an extremely rare but serious allergic reaction to the vaccine.

And vaccines can trigger rare but serious reactions, even among people with no apparent allergies or sensitivities.

If rare reactions occur with any product, they may not be identified until thousands, or millions, of people have used it.

It also required both partners to take responsibility for a shared experience and had no side effects, except for a rare allergic reaction.

Clinical trials are not large enough to find such a rare reaction.