Naloxone is a pharmaceutical that acts as an opiate blocker. It is commonly used to treat narcotics overdose as it binds to opiate receptors in preference to narcotic drugs. When treated with naloxone, the effects of opiate overdose such as suppression of the heart and lungs are instantly reversed. However, it also negates the pain reducing effects of narcotics as well.
In Fools for Love, House gives naloxone to Jeremy in order to put him in pain and convince him to allow a brain biopsy on his wife Tracy. It doesn't work and Foreman has to use a sedative to relieve the pain.