Analgesic is defined as medicines used to provide pain relief. They are also known as painkillers. According to the website Drugs.com, the term analgesic refers to a medication that relieves pain without loss of consciousness, as opposed to an anesthetic, which is a substance that induces insensitivity to pain via a loss of consciousness and an absence of sensory perception. A large number of medicines have pain-relieving properties, and analgesics with similar mechanisms of action are usually grouped together; for example, nonsteroidal anti-inflammatory drugs, or NSAIDs, and narcotics. Analgesics can also be grouped depending on the severity of pain they are indicated for, for example, acetaminophen and NSAIDs are indicated for mild-to-moderate pain, and weak opioids, such as codeine, dihydrocodeine or tramadol are indicated for moderate-to-severe pain. Central nervous system analgesics act in the brain or spinal cord to produce their effects; examples include opioids such as morphine and oxycodone. Peripherally acting analgesics act outside of the brain and spinal cord and include NSAIDs and COX-2 inhibitors. There are a plethora of analgesics including acetaminophen, or Tylenol, that do not require a prescription. Some medicines combine opioids with acetaminophen in order to ensure relief. In order for analgesics to work, they bind to receptors on cells mainly in the brain, spinal cord and gastrointestinal system. As previously stated, there are a variety of classifications for analgesics such as paracetamol/acetaminophen, NSAIDS, COX-2 inhibitors, and opioids and each one has a specific use for a specific type of pain. For example, for a mild to moderate pain, paracetamol/acetaminophen are used to treat pain along with fever. Medications like Advil and Tylenol are within this classification group. NSAIDs are used for similar uses, to reduce fever and pain. Medications like Ibuprofen are within the NSAID classification group. Derived from NSAIDs, were COX-2 inhibitors. They are used for ailments similar to those of NSAIDs, however research has shown that most of the drugs classified within this class lead to an increased risk of cardiovascular events by an average of 40 percent. Finally, opioids, medications within the opioid classification are the strongest analgesic compared to the other three classifications in that their side effects are more severe. Opioids, while very effective analgesics, may have some unpleasant side-effects. Patients starting morphine may experience nausea and vomiting. Constipation occurs in almost all patients on opioids, and laxatives are typically co-prescribed. When used appropriately, opioids and other central analgesics are otherwise safe and effective, however risks such as addiction and the body's becoming used to the medication can occur. The effect of tolerance means that frequent use of the drug may result in its diminished effect so, when safe to do so, the dosage may need to be increased to maintain effectiveness. This may be of particular concern regarding patients suffering with chronic pain. Opioid tolerance is often addressed with opioid rotation therapy in which a patient is routinely switched between two or more non-cross-tolerant opioid medications in order to prevent exceeding safe dosages in the attempt to achieve an adequate analgesic effect.