Cisplatin, known chemically as cis-dichloro-diaminoplatinum, is a powerful antineoplastic drug; that is, a drug which kills cancer. Unfortunately, it does quite a number on the rest of you, too. It is ototoxic and can cause balance problems and hearing loss. These side effects are largely irreversible. It is also extremely emetogenic. Somewhere around 98% of patients treated with cisplatin experience severe vomiting attacks and debilitating nausea. To combat this, patients are premedicated with ondansetron, dexamethasone and sometimes dronabinol.
Cisplatin was discovered serendipitously when an experiment studying the effect of electric currents on bacterial growth went wrong. The experiment used platinum electrodes to minimize interference. Ironically, this caused a chemical reaction which /did/ interfere with the experiment. When the culture was analyzed, two chemicals were isolated from it, trans-dichloro-diaminoplatinum and cis-dichloro-diaminoplatinum. The former chemical, the trans isomer, later proved to be horribly toxic, so it was the cis isomer that was adopted as the chemotherapy drug.
Some variants, such as carboplatin have been developed, which have a lower occurrence of side effects (carboplatin, specifically, has a side effects profile similar to doxorubicin). Despite this, they are not always appropriate, since cisplatin has much greater activity against some cancers than carboplatin does.