A common practice amongst street drug manufacturers and dealers is to combine the illicit drug of abuse they intend to distribute with additional substances. Substances added in an attempt to mimic or enhance the desired effect of the drug of abuse are known as adulterants as opposed to diluents, which are added simply to increase the weight of the product. By definition, an adulterant has a physiological effect on the body and as physiologically active compounds these adulterants may have side effects apart from the drug it is added to. They can be minimal, treatable, or otherwise manageable while others can be worse than those incurred from the drug of abuse itself. Due to varying trends in street drug purity, an effort must be made to understand the effects adulterants and diluents, as well as mixtures thereof, may have on forensic drug analysis. Colorimetric spot test analysis is typically performed using a representative sample of the raw, suspected drug material. These tests are often employed in the lab prior to any attempt to isolate a specific compound or at the scene before an arrest is made. This being the case, the reagents will be exposed to and have the potential to react with anything present in the sample with the drug of abuse. While much work has been done regarding the specificity and cross reactivity of colorimetric spot tests, limited information is available about how mixtures of adulterants may affect the results. This research consists of two parts which approach the common theme of adulterants differently. The first part is a literature based investigation into the pathological side effects of several common drug adulterants. The specific compounds discussed are: levamisole, phenacetin, atropine, and several topical anesthetics (benzocaine, lidocaine, prilocaine, and procaine). A review of articles from the scientific literature was conducted in order to convey what is known in the medical field regarding the effects these compounds can have on the body. The second part of this research was a laboratory based investigation which analyzed the effects of twenty-three common adulterants on two colorimetric spot tests: the Marquis reagent for the presumptive identification of heroin, morphine, amphetamine, methamphetamine, and 3,4-methylenedioxy-N-methamphetamine (MDMA) and the modified Scott test for the presumptive identification of cocaine. This was performed in order to observe the reactivity of these compounds so that a better understanding of the effect their presence can have on the analysis of seized drug samples using these tests could be obtained. The literature review portion of this research revealed that the adulterants levamisole, atropine, phenacetin, and the topical anesthetic adulterants lidocaine, prilocaine, benzocaine, and procaine, can be toxic and have severe, deleterious effects on the body in both chronic and acute exposures. Levamisole stimulates the immune system resulting in the production of self-reactive antibodies that attack neutrophils. This causes an autoimmune disorder that weakens the immune system and causes leukocyte agglutination leading to necrotizing vasculitis. Atropine functions to decrease the parasympathetic tone and increase the sympathetic tone in the body. An overdose can cause anticholinergic toxicity, a syndrome very similar to sympathomimetic toxidrome caused by cocaine overdose. Both are characterized by hypertension, hyperthermia, tachycardia, ataxia, disorientation, and mydriasis. However, they can be distinguished as anticholinergic toxicity causes dry and flush skin and mydriasis which is unreactive to light while sympathomimetic toxidrome causes profuse sweating and mydriasis which is reactive to light. Phenacetin is metabolized to O-ethyl-N-acetyl-p-benzoquinone imine (O-Et-NAPQI), a highly reactive and unstable electrophile. It is capable of covalently binding with proteins and other cellular components, including deoxyribonucleic acid (DNA). This leads to mutagenesis and subsequent tumor generation as well as apoptosis and necrosis of various tissues in the bladder and kidneys. The topical anesthetics have each been associated with the development of methemoglobinemia. This is a condition caused by an increase in the concentration of methemoglobin in the blood. Methemoglobin binds more strongly to oxygen so that it cannot be released to the tissues resulting in oxygen starvation. This was found to be caused by the metabolism of prilocaine and lidocaine to the oxidative compounds O-toluidine and 2,6-xylidine respectively. The cause has not been previously reported for benzocaine or procaine. The laboratory portion of this research revealed the impact several adulterants and their mixtures can have on colorimetric spot tests. Testing with approximately 1 mg of sample material revealed that eight out of the twenty-three adulterants reacted with the Marquis reagent to cause a color change within 15 minutes. Of the ten 3-component mixtures tested, eight resulted in a color change. Of the six five-component mixtures, four resulted in a color change; of the six eight-component mixtures, three resulted in a color change; and of the six ten-component mixtures, five resulted in a color change. Of the color changes observed, none were consistent with the "expected" color change for a presumptively positive result of the Marquis test. Testing with the adulterant diphenhydramine revealed that this compound had a unique and intense reaction with the Marquis reagent. This compound initially reacted by turning the liquid a vibrant yellow-green upon contact with the sample. Prior to 2 minutes, solid red-brown aggregates formed in the liquid and proceeded to increase in size while the reagent solution darkened to a red-black color over the 15 minutes of observation. Mixtures containing diphenhydramine each reacted differently. Solid material did not form in the mixture tests and the color changes observed ranged from light orange to a dark red-orange. In addition, one 3-component mixture containing adulterants which did not cause a color change when tested individually (diltiazem, acetaminophen, and quinine) was observed to cause a light brown color to develop, which darkened over the 15 minutes of observation. The results for the modified Scott test showed that the complexity of this test has made it highly specific for cocaine. None of the individual components or mixtures tested reacted in a manner consistent with a presumptively positive identification for cocaine hydrochloride (HCl) or cocaine base for all three steps. Of the twenty-three adulterants tested, eight of the twenty-three gave a similar result to cocaine base in step 1, however, of these eight, only one (quinine) gave a similar result to cocaine base in step 2, and this adulterant did not react consistently to cocaine base in step 3. The research using the adulterant mixtures revealed that the compounds present in these samples tended to react individually with the modified Scott test reagents. In other words, the results for each mixture appeared as a combination of the individual results for each component. While the results observed for the adulterants and mixtures tested were not consistent with the expected presumptively positive results for the drugs these tests are used to detect, it was concluded that the presence of adulterants in a sample has the potential to affect the results of a colorimetric spot test in a variety of ways. Given that these tests are typically employed on samples of raw suspected drug material, it is important that analysts understand the impact adulterants can have on the interpretation of presumptive drug tests.