Medical kits come in many different shapes, sizes and qualities. Each kit contains different things, and should ICly enable you to perform different tasks. The capabilities of your kit will vary depending on its quality and should be roleplayed according to what would be in it. While some license is given in this, its fair to assume you won't have advanced drugs, morphine, etc. in a Fair medical kit or below. Likewise, you won't have huge amounts of high end drugs or advanced tech scanners in anything under Superb.
Medical kits are useful field tools, but they're often incapable of doing anything more than reducing the likelihood of the -increase- in a wound's potential danger. They're no substitute for a Doctor's office or surgical suite. They can basically stop injuries from getting worse. Unless you're using one of the really high end ones, don't expect them to do anything more than reduce your patient's suffering, or keep a scratch from turning into a more serious infection or wound.
This is, of course, if you're just using the kit to patch wounds. The effects of giving shots of tranquilizer to reduce panic or morphine to reduce pain should always be RPed out, even if they end up having a coded effect (as is to be determined by the admin running the scene, if any, or all players there, with consent). This is an opportunity for fun and such, but be reasonable. People don't pack virulent poison in medkits, so you can't run around using its contents as an injected weapon. Besides, injecting someone with a big 'ol syringe full of air is generally more deadly.
Also, in the case of certain medical kits and the supplies within them, you need clearance from a government agency to cart around their potentiall dangerous contents. Syringes can be considered weapons, and certain drugs are illicit on some worlds. For example, anyone who's not a doctor, or doesn't have some official reason from a planet's government, will be in quite a bit of trouble if found carrying around a Great medkit stuffed with morphine.
Mediocre or worseEdit
A medical kit at this level is basically gauze and a splash of disinfectant. While you might be able to competently reduce the impact and festering quality of wounds, don't bet on it. You can just about tie a tourniquet, and it's likely to be with dirty cloth. The disinfectant will be harsh, and it will -sting-.
This is the kind of medkit you'd be used to seeing at home. It's probably got gloves, gauze, a tight kind of wrap, gentle disinfectant, cotton balls, and other small necessities. What it does not have is any kind of syringe or drug. It does not contain pills of any sort. It contains a few mundane, physical items, along with medical tape and very sharp scissors. This is all. This is also likely the highest type of medkit you can can carry without clearance or authorization from planetary governments, with a few exceptions. The reason for this being that all higher-level medkits contain drugs or more advanced technology.
Good and GreatEdit
Any medkit above Fair can technically be considered a "Doctor's Bag". The difference between these medical kits and lower-level ones lies in their content of liquid drugs and hypodermic needles. A Good medkit might have a low-dose sedative to abate panic attacks or a slight stimulant to keep a combat medic's patient on his feet for a while longer. It might also contain light painkillers (stronger than tylenol, weaker than ketamine). A Great medkit will likely contain greater dosages, as well as more esoteric additives like clotting salves to close wounds almost instantly, or disinfectant strips that, in a stark miracle, -don't- sting like shear hell.
Now we're getting to the good stuff. Superb medical kits are so good you can perform <Species> Medicine rolls with them. Granted, at a penalty, but they're still damn good. The medkit boxes themselves, or a small device within them, generally comprise a drug repository for tiny compressed cartridges of myriad types. These may contain nigh-instant (but limited duration), skin and tissue regenerators, instantly bonding and clotting styptics, and much more. The exact effects will vary, but they're rare and powerful. At this level and above, needles are likely replaced with more efficient pneumatic decompression devices which inject with less pain, and no protruding metal pointy bits.
A legendary medkit would basically be a fully operational doctor's office. It possibly takes advantage of spatial bends to create enough room for all of the things it must be good for. It can also likely keep a bad wound from getting worse. The abilities of a medkit are still limited within reason, and you obviously can't perform surgery with one, no matter how miraculous its technologies, drugs, or bandages.