Injectable MTX comes in 2 forms, with preservatives or preservative free. No matter if you have a 10ml vial or a 2ml vial you can't go by quantity for how long this medication will last.
MTX preservative free is one time use. Without the preservatives, once you puncture the rubber, the medication is compromised. It starts to grow bacteria.
MTX with preservatives is good for 28 days after puncturing the rubber. After 28 days bacteria starts to grow also.
So let's say you inject 1ml a week. You have a 10ml vial with preservatives. Doing the math quick you would think you have a 10 week supply. That is incorrect. You have a 4 week supply and you must discard 6ml. If you keep using it then on weeks 5-10 you are injecting a medication with bacteria that won't work as well.
Having a compromised immune system plus injecting bacteria could have very serious consequences. Please pay attention to your vials and this detail. Below are the different forms MTX can come. You will see under the pictures it will tell you if it's preservative free or not.
Knowledge is power. I'm not doing this post to knock MTX because for me, this medication has changed my RA for the good. I'm just spreading awareness.
*MTX picture courtesy of epocrates apple app