If your tarantula bites you, which is likely only if you try to handle it or are careless when cleaning the cage, you could suffer some pain and possible bleeding because the tarantula basically has driven small nails into your hand. The pain soon passes, but the area around the bite may become red and slightly swollen, and there may be a numbing tingle in the hand or adjacent fingers for a few hours to a day.
Keep the wound iced and apply an antibiotic salve to prevent secondary infections. That’s usually about it—no fingers falling off, no heart attacks, no need for a visit to the emergency room. This applies to almost all common pet tarantulas, especially the species of Aphonopelma, Brachypelma, Avicularia, and Grammostola found in pet shops.
If you are bitten by one of the potentially dangerous South American, African, or Asian tarantulas, it would be wise to spend the rest of the day just reading or watching TV and avoiding exercise that could spread venom. If you feel any shortness of breath or notice an irregular heartbeat, have someone calmly drive you to an emergency room for observation. Almost certainly nothing will happen, but one never knows for sure. Individual reactions to any venom (from bees to rattiesnakes) vary greatly, and self-induced hysteria over the potential danger of a bite could be worse than the venom itself. Remember, there is no hard evidence that any tarantula in captivity has actually killed anyone, and most bites don’t even inject any venom (so-called dry bites).
Avoid speculative treatment for bites. At the moment, there is no antidote for a venomous bite and very little evidence to show that one is needed. There also is no real evidence that the venom of a big Theraphosa spp. is the same as that of a feather-leg or that either would react well to injections of widow spider (Latrodectus spp.) antivenin.
Treatment, if any, presumably would have to be supportive to stabilize breathing and heartbeat. I’m not aware of any such treatment ever being needed in theUnited States, though thousands of people keep tarantulas, including many of the potentially dangerous species, and bites are not uncommon.