Chronic, repeated use of a substance over a period can lead to tolerance and dependence on that substance. Tolerance and dependence are significant factors in addiction and recovery as both offer serious implications for the physical and mental well being of a person.
Cross tolerance is a phenomenon where a person who has developed a tolerance to one drug also has a certain level to drugs in a similar class. An example of this phenomenon is apparent in the development of a high tolerance to the stimulant amphetamine which can result in a tolerance to methamphetamine because it is also a stimulant and has a similar structure chemically.
Cross dependency is an associated issue that some addicts may experience where their dependence on a drug can be alleviated by the use of another drug. Instances occur where individuals may begin to use drugs that are chemically different and produce different bodily reactions to the drug they are addicted to, and they feel none of the affects of withdrawals. This can occur with individuals who are dependent on smoking crack cocaine but can switch to heroin to meet their needs. This may be because an addict has different brain chemistry and are hyper-sensitive to addictive substance.
Tolerance and dependence occur after long term, chronic use of a substance. With increasing tolerance to the drug, more is required to get the same effect. Tolerance and dependence develop as the nerve cells chemically and structurally counteract the drugs effects.
Dependency is the persistent, compulsive and repetitive use of a substance that results in tolerance. Dependence on a drug can be both physical and/or psychological. Some drugs such as heroin and alcohol will cause a person to exhibit painful physical symptoms when they are not using the drug or if they attempt to stop using the drug without medical support. Cocaine addicts will often experience the intense cravings associated with their dependence for months and sometimes years.
Tolerance occurs in the brain through a complex chain of molecular and receptor changes. When a drug is in the body and stimulates the release of dopamine from the dopamine receptors in the brain. When the receptors are stimulated a person will experience feelings of pleasure and well being. But if a person uses the drug continuously, the receptors will be hyper-activated and the body will make changes to adjust the levels of chemicals in the body that may cause harm. The brain can turn off some of the receptors in the brain that are being stimulated to counteract the effects of the drug which results in a tolerance to the drug. A person can continue to use the drug at the same level but not feel the effects.
Drugs are grouped into different classes according to their effect on the central nervous system, for example opioids, stimulants, or psychoactive drugs. The body can build up a tolerance to the effects of a drug and require more of the substance to get high. As this tolerance builds, it can expand to other drugs in the same class. Studies have shown that people who smoke cigarettes experience a cross tolerance to caffeine and report symptoms of using caffeine that are significantly less than those who do not also smoke cigarettes. Some pharmaceutical drugs also have risks of cross tolerance such as benzodiazapine and anti-depressant drugs due to their similar effects.
Some pharmaceutical drugs are similar in chemical makeup to illicit substances. This can cause some cross tolerance and dependence issues. Adderall, Ritalin and Dexedrine are all drugs that are commonly prescribed but they are all based on an amphetamine chemical structure. For this reason, cross tolerance is possible, especially with use of these prescription medications for over 6 months. Morphine and heroin are related drugs which also have cross tolerance issues which is why morphine, the pharmaceutical opiate, is heavily regulated in most countries.
Alcoholics can experience problems with cross tolerance when they are undergoing treatment that includes the use of medications. Benzodiazapine drugs are often prescribed to people who suffer alcoholism and depression but there are concerns about the cross tolerance to the effectiveness of benzodiazapine for alcoholic. This is because benzodiazapine medications and alcohol are both central nervous system depressants
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