Using healing serum on my character

I made the serum, I keep it handy, but nothing I do enables me to use it. I've tried T, G, etc. Changing assigned keys, etc. Nothing. ARGH. I am playing on PC. Please help?

You need to have less than full health to be able to use the healing serum, is this the case?

Hi, Yes I am near empty on health. Thank you.

Have you managed to use it before, or is it impossible to use at all?

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