If a drug is being sold illegally, chances are its sellers have added crap to it in order to make more money. Cocaine is no exception to this deception. Substances added to nose candy because they resemble the drug but otherwise don't mimic or influence its effects include talc powder, flour, cornstarch, inositol and other sugar alcohols, various salts, boric acid, and microcrystalline cellulose. Snorting any of talc, starch, or cellulose into your lungs can result in an inflammation that disrupts your ability to breathe normally.
|A decidedly artsy photo of talc powder between two brushes (Source)|
There are also a bunch of drugs known to be mixed in with cocaine to dupe a buyer into believing they're purchasing a higher quality product. Caffeine is used in this capacity because it's cheap and is a stimulant (it perks you up) like cocaine, albeit a way less intense one. Several relatives of cocaine, including procaine, lidocaine, and benzocaine, are used as local anesthetics to do things like numb your mouth at the dentist before the drilling commences. Cocaine also causes mouth numbness, so adding these other drugs to it can trick customers into thinking they're getting a higher quality product.
One of the stranger yet very common additions to cocaine is a drug called levamisole. It's good at killing parasitic worms and also appears to be able to influence the immune system in useful ways. Unfortunately, it also has a tendency to ruin bone marrow and cause a serious dip in the number of white blood cells being made there (agranulocytosis), which is a fantastic way to catch a life-threatening infection. Snorting cocaine contaminated with levamisole can also damage blood vessels in the arms and legs via vasculitis. The role of levamisole as a cocaine adulterant is a bit of a mystery. It's apparently transformed into aminorex, an amphetamine-like drug, when given to racehorses, suggesting it can enhance the effects of cocaine. There's also some evidence that levamisole can act on receptors in the brain to enhance how good cocaine makes you feel. Alternatively, it may simply be added because it's cheap, resembles cocaine, and is often easy to acquire since it's sold as a veterinary medication.
Our tour of drug additions to cocaine ends with diltiazem. This drug acts to block the movement of calcium through channels in the outer membranes of heart cells, permitting it to be used to help control an erratic heartbeat (arrhythmia) and other heart issues. It's been suggested that some distributors decided to add diltiazem to their cocaine in order to negate the negative effects it can have on the heart, which happen to include arrhythmias. Even so, there isn't any evidence diltiazem can provide a protective effect. If anything, it might actually make things worse.
Brunt TM, Rigter S, Hoek J, Vogels N, van Dijk P, Niesink RJ. 2009. An analysis of cocaine powder in the Netherlands: Content and health hazards due to adulterants. Addiction 104(5):798-805.
Cole C, Jones L, McVeigh J, Kicman A, Syed Q, Bellis M. 2011. Adulterants in illicit drugs: A review of empirical evidence. Drug Testing and Analysis 3(2):89-96.
Tallarida CS, Tallarida RJ, Rawls SM. 2015. Levamisole enhances the rewarding and locomotor-activating effects of cocaine in rats. Drug and Alcohol Dependence 149:145-150. [Full text]