All sorts of closures for the glass vials were considered for antimicrobial protection. Rubber closures actually allow ingress of contamination from the hole through which the needle passes, because that hole is too large to keep out bacterial contamination. There are multiple studies reporting that. Here’s one. Also, rubber closures are prone to sample contamination by rubber additives. Here’s another study for your information. This is more well-known now and another reason to avoid that type of closure, although there are still plenty around, as they tend to be cheaper. To maintain the integrity of your methyl B12, ensuring the vial remains tightly closed is the most important thing. And, it is actually the staph aureus on our skin that causes most problems, so always clean your skin with alcohol swabs or soap before injecting.