I've always been curious as to why some medications are administered in tablet form while others are given by injection. Can you explain why?
Most medications have to be absorbed into the bloodstream for them to reach the site where they are needed. The method by which a drug is introduced into the body will influence how it gets into the bloodstream and how quickly it will work.
Drugs can be administered in a variety of ways: by mouth, rectum, injection or by inhalation. In addition, slow release drugs can be implanted under the skin and enter the bloodstream in a sustained way over time.
Where it is undesirable for large amounts of a certain drug to enter the body it may be applied topically to the skin so as to limit its effects to the site of the condition. Injected drugs can be delivered into a vein (intravenous), directly under the surface of the skin (subcutaneous) or into a muscle (intramuscular). Many drugs are made in different forms to facilitate their administration.
What about drugs given rectally?
Drugs that are used for their general, systemic effect are often given per rectum. By inserting the suppository or enema form of the drug into the rectum, it is absorbed quickly into the bloodstream. This method is often used for medications that can be destroyed or altered by the stomach's digestive juices. It's also a useful method to use where a person has difficulty swallowing or is suffering from nausea and vomiting. Some drugs are given in this way to act locally as in the treatment of haemorrhoids or ulcerative colitis. In France, medications are often prescribed in their suppository form.
You mentioned inhaling drugs, how do they work?
Inhaled drugs can be used for general effect throughout the body or direct local effect on the respiratory tract. When general anaesthesia is administered through inhaled gases, these pass through the lungs and into the bloodstream producing general effects throughout the body and brain. Other drugs, such as those prescribed in the treatment of asthma, bronchitis or emphysema, are inhaled into the lungs and not only reach the bloodstream this way but also have a direct effect on the lungs themselves.