There are any number of reasons an IV could be used. As somebody pointed out earlier, it keeps an open line for drugs, for taking blood, and for replacing fluids (Blood or saline). It's possible Amy had lost enough blood from her scratches that
Tanya was giving her saline or Ringer's solution to replace at least some of her volume. As far as drugs go, some drugs can only be administered by IV or that the only formulations they have left at
the colony are IV formulations of drugs that also have an oral form. If they are as low on supplies as indicated before, it seems reasonable to me that they would have used the oral drugs previously, holding onto the IV formulations for a worst case scenario which we are now starting to reach.
That said, I wouldn't be surprised to find out Tanya had been hoping to keep Amy from turning, and thought she could while keeping everybody else safe. I don't think she would put everybody at risk-especially because she would know in a triage situation like this that she wouldn't be able to keep as close an eye on Amy as she would need to.