Antivenom is used for curing snake bites. Is it fatal when used without being bitten by a snake?
I'd like to preface this by saying don't randomly use medicines without a doctor's advice. It's fairly unlikely antivenom would cause you any harm but in some cases, antivenom could be dangerous and even lethal. You have to consider possible allergic reactions and the route of administration. In any case, it would be a stupid idea to blithely inject yourself with antivenom for no reason, so why run the risk?
2. Introduction and Mechanism
Antivenom is made of a type of protein called an antibody. Antivenom acts by chemically binding to the animal's venom in such a way that the venom is blocked from binding to anything in your body. This prevents the venom from causing any damage. In fact antibodies typically bind very specifically, so they wouldn't usually bind to anything other than the venom itself. Hence, it's unlikely that antivenom would bind to anything in your body or cause any off-target effects if they were injected into your body.
But antibodies are all proteins, and your body will be skeptical about any foreign protein since it could signal a pathogen. Hence, an injection of antivenom could cause an immune reaction and possibly lead to anaphylaxis. Anaphylaxis is potentially fatal, and it wouldn't be easy to know whether your body will have a reaction to the antivenom so basically keep random proteins out of your bloodstream.
4. Route of Administration
The way you introduce the antivenom to the body will have an impact on its effect. Consider what happens when you eat something. Your body passes the food from your mouth to your stomach, then liver and so on. At each step, the food is processed and broken down. In fact, chemicals like proteins are broken up during digestion, so they typically lose their effects. Hence, the effect of the antivenom will depend on where in this chain it enters the body, which determines how much it's broken down. An intravenous injection of the antivenom could have strong effects that you might be safe from, if you eat the antivenom. This is similar to the adage that you can potentially eat a snake's venom and not get harmed.
This is the reason antibody-based drugs are typically injected. By avoiding proteolysis of the antibody, it retains its shape and can successfully bind to its epitope.
Your use of the word "cure" in the title is arguably slightly suspect. Antivenom doesn't stop or reverse the effects of venom that's already bound to chemicals in your body, antivenom just stops more venom binding.
Anti-venoms are not fatal. They are also ideally not fatal even if administered without any snake-bite, provided the anti-venom has been approved for clinical use by the pharmaceutical regulatory board for the country in question. World-wide, the Food & Drug Admin. (FDA of the USA) standard of phases of clinical trials is generally used or followed.
Anti-venoms become fatal if the person given the drug develops a hypersensitivity reaction to the drug, that is why a skin test using a small amount of the drug is injected intradermally on the forearm is conducted prior to giving the full dose of the drug. This way, toxicity is minimized, usually by not giving the drug when a positive test is unanimously declared by clinic staff.
Simplistically, a good rule of thumb to consider is that all drugs without exception are potentially toxic. That is why drugs are considered double-edged swords. Having accepted that, first, a drug should not cause someone to be sick if given to a healthy person. The related reasoning goes that if a drug causes a healthy person to be sick, then the more a sick person given the drug will more likely feel sick, i.e. 'sicker' than previously when he had no drug treatment. That is embodied in the Phase I clinical trial, where it is implied but not stated the Hippocratic oath 'First do no harm.' Hence, the subjects for that phase are (classically) generally healthy college students. By phase II, (classically) sick patients are added to the test population. By phase III, usually more hospitalised patients are included. By phase IV, generally out-patients become enrolled in the study, and so on.
For the drug company to recoup its investment in drug development, then a suitable sponsor agreement may be undertaken to administer the drug for free, and also to keep the drug industry afloat to develop more drugs.
Well-known snake venoms are phospholipases. Lipases are enzymes that can specifically target the lipid bi-layer of cell membranes of tissues and cause them to disrupt. Most anti-venoms are also enzymes that destroy these foreign lipases, and their toxicity may lie in their ability to disrupt our own lipases needed for critical bodily functions.