I use one of these safety lanyards from EK USA:

Safety Lanydards

They all have a safety breakaway and are of high quality. You can find them cheaper elsewhere, such as Ebay.

As to the Medical Alert: I think the above lanyards are a little too beefy to wear all of the time and you want to wear your Medical Alert tag all of the time. If you go with a beaded chain, make sure it is stainless steel and not nickle, as many people have allergies to nickle.

Pete