In common with all narcotic analgesics, codeine  has a large number of adverse effects. The most common adverse effects are constipation and sedation. Because codeine is a narcotic, it may be addicting, and is rigidly controlled under United States and international law. Although it has been widely abused in past years, particularly in the form of cough syrups, codeine itself appears to have declined in use and has been replaced by similar, but more powerful drugs.

"Codeine." Drugs and Controlled Substances: Information for Students. Ed. Stacey L. Blachford and Kristine Krapp. Detroit: Gale, 2010.Opposing Viewpoints in Context. Web. 16 May 2016.

Although codeine is present in nature within the sticky substance latex, which oozes out of the opium poppy’s seed pod, it is present in only small concentrations (less than 0.5). However, morphine, another opioid analgesic, is present in greater concentrations (10) within the opium poppy’s latex, and codeine can be made from morphine via a process known as methylation, which is the primary way that codeine is prepared.

Codeine is a centrally acting drug, meaning that it goes to specific areas of the central nervous system (in the brain and spinal cord) to interfere with the transmission of pain and to change perception of the pain. For example, if you have your wisdom teeth removed and your mouth is feeling very painful, codeine will not go to the hole in your gum where the tooth was pulled and which is now throbbing with pain, but rather will act with the central nervous system to change the way you are perceiving the pain. In fact, if you were given codeineafter your tooth was pulled, you might explain its effect by saying that the pain was still there, but it just was not bothering you anymore.

Codeine’s anti-tussive (cough stopping) effects are also due to its central actions on the brain. It is believed thatcodeine inhibits an area of the brain known as the medullary cough center.

Codeine is not as potent a drug as is morphine, so it tends to be used for only mild-to-moderate pain, while morphine is useful for more severe pain. An advantage to using codeine is that a significant degree of pain relief can be obtained with oral medication (taken by mouth), rather than by injection. Codeine is sometimes preferred over other opioid analgesics because it has a somewhat lower potential for addiction and abuse than do other drugs in that class.

"Codeine." The Gale Encyclopedia of Science. Ed. K. Lee Lerner and Brenda Wilmoth Lerner. 4th ed. Vol. 2. Detroit: Gale, 2008. 985-986. Web. 16 May 2016.

People experiencing levels of pain that will not respond to over-the-counter drugs can sometimes receive prescription pain relievers containing various dosages of codeine. Tylenol-brand acetaminophen is one product that contains codeine in its prescription form. These drugs are assigned the numbers one through four to indicate their various strengths. Here's a quick rundown of "Tylenol by the Numbers":

"Codeine." UXL Encyclopedia of Drugs and Addictive Substances. Ed. Barbara C. Bigelow. Vol. 2. Detroit: UXL, 2006. 186-99. Opposing Viewpoints in Context. Web. 16 May 2016.

It remains a misconception that opioids offer no true analgesic effect, but instead produce a type of euphoria that simply results in one not caring about their pain. With the discovery of opioid receptors in the central nervous system, along with an understanding of how opioids such as codeine affect nerve cells, no dispute remains that opioids are indeed potent pain relievers. The more codeine ingested in a single dose, the greater the analgesia. People with moderate or severe pain usually don’t report feelings of euphoria. Those with mild pain who take one of the higher doses of codeine (e.g., Tylenol #4) may experience pain relief along with some euphoria. People who abuse codeine are most likely to experience euphoric feelings, and are at the greatest risk of becoming addicted.

"Codeine." Drugs and Controlled Substances: Information for Students. Ed. Stacey L. Blachford and Kristine Krapp. Detroit: Gale, 2010. Opposing Viewpoints in Context. Web. 16 May 2016.

Other than analgesia, the most common physiological effects produced by medicinal quantities of codeine are nausea, vomiting, constipation, and itching of the skin (pruritis). For most people, the higher the codeine dose, the more likely side effects will be present or the more severe the side effects will be.

Overall, codeine's lower potency results in fewer side effects compared to other drugs in the class. The complication of greatest concern is respiratory depression. Opioids affect the part of the brain that controls breathing, and a large enough dosage can halt breathing and result in death. Combining codeine with another central nervous system depressant, such as alcohol or sedatives, is especially risky. Drowsiness caused by the opioid can be a problem if a user decides to operate a vehicle while intoxicated or under the influence. Most codeine is usually part of a compound preparation, which is why it can have many side effects. Someone with stomach ulcers should not take codeine that is combined with a nonsteroidal anti-inflammatory drug (NSAID) such as aspirin or ibuprofen. This can also lead to abuse. For instance, a person who abuses codeine might routinely take a dose of 100-200 mg of codeine to produce noticeable euphoria. Using Tylenol #3 to obtain this dose would also mean ingesting 1,000-2,000 mg of acetaminophen. Taking that amount of acetaminophen for any extended period presents a risk for liver damage, especially in combination with alcohol.

"Codeine." Drugs and Controlled Substances: Information for Students. Ed. Stacey L. Blachford and Kristine Krapp. Detroit: Gale, 2010. Opposing Viewpoints in Context. Web. 16 May 2016.

"Codeine." Drugs and Controlled Substances: Information for Students. Ed. Stacey L. Blachford and Kristine Krapp. Detroit: Gale, 2010. Opposing Viewpoints in Context. Web. 16 May 2016.

"Codeine." Drugs and Controlled Substances: Information for Students. Ed. Stacey L. Blachford and Kristine Krapp. Detroit: Gale, 2010. Opposing Viewpoints in Context. Web. 16 May 2016.

Codeine is used to relieve mild to moderate pain. It is also used, usually in combination with other medications, to reduce coughing. Codeine will help relieve symptoms but will not treat the cause of symptoms or speed recovery. Codeine is an opiotic, and when codeine is used to treat pain, it works by changing the way the brain and nervous system respond to pain. When codeine is used to reduce coughing, it works by decreasing the activity in the part of the brain that causes coughing.

Codeine is also available in combination with acetaminophen (Capital and Codeine, Tylenol with Codeine); aspirin; and carisoprodol; and as an ingredient in many cough and cold medications. 

Codeine (alone or in combination with other medications) comes as a tablet, a capsule, and a solution (liquid) to take by mouth. It is usually taken every 4 to 6 hours as needed. Follow the directions on your prescription label carefully, and ask your doctor or pharmacist to explain any part you do not understand. Take codeine exactly as directed. Codeine can be habit-forming. Do not take a larger dose, take it more often, or take it for a longer period of time than prescribed by your doctor. If you suddenly stop taking codeine, you may experience withdrawal symptoms such as restlessness, widened pupils (black circles in the center of the eyes), teary eyes, irritability, anxiety, runny nose, difficulty falling asleep or staying asleep, yawning, sweating, fast breathing, fast heartbeat, chills, hair on your arms standing on end, nausea, loss of appetite, vomiting, diarrhea, stomach cramps, muscle aches, or backache. If you are giving codeine to a child, give the medication only as needed. Do not give codeine on a regular (around-the-clock) schedule and do not give more than six doses in 24 hours.

"Codeine: MedlinePlus Drug Information." U.S National Library of Medicine. U.S. National Library of Medicine, n.d. Web. 18 May 2016. <https://www.nlm.nih.gov/medlineplus/druginfo/meds/a682065.html>. 

"Codeine." Drugs and Controlled Substances: Information for Students. Ed. Stacey L. Blachford and Kristine Krapp. Detroit: Gale, 2010. Opposing Viewpoints in Context. Web. 16 May 2016.