POSSIBLE SPOILER

When you fight gray fox in the lab of the nuclear warhead storage building, at some part he screams, "the mediciiine..!". What is this? Is he talking about Fox-die? Or when they did experiments on Fox?

fox's body rejects the exoskeleton because it was forced on him and stuff(i can't really explain very well but the suit is basically his new body but his real human body of flesh and blood doesn't want it),so it causes massive headaches and he goes on a rampage jumping around and cutting anything around him,I'm guessing that "the medicine" he was talking about may reduce the pain of his headaches or something :o

They used the dead body of Frank Jaeger after Snake killed him to research exoskeletons and performance enhancing medicine, and kept his body alive but kind of in a coma, while they "experimented on him like a plaything", testing out drugs and chemicals, and some worked, giving him his abilities, but them made him unstable (his one source of stability was Snake), and I think the exoskeleton keeps his body going, because without it his body would shut itself down due to the drugs etc that they pumped into him, thats why Fox wants to die.

the medicine is to prevent him from going lunatic, because he is unstable.

What? The medicine made him go mad, he was in control of himself and then shouted "The Mediciiiine." The same happened when he was with Snake after their fight, he lost himself, started doing alll the flips and disappeared, just like he did after he cut Ocelot's hand off. That's how it looks to me anyway...

The medicine is there for the exoskelaton and obviously not Foxdie because why would Naomi program the virus to kill her "brother" plus, she didnt even know he was going to show up.

The medicine works the way it should when anything is implanted into your body. Even in heart transplant surgery you need to be one some sort of drug to keep your body from rejecting the new heart ergo Fox takes to medicine to keep his body from rejecting the suit and killing him.