Bioavailability of drugs refers to the amount of drug that actually reaches systemic circulation unchanged or in its active form. People often study the bioavailability of drugs through different routes of administration. By definition, intravenous administration has 100 percent bioavailability. However, some drugs must be broken down by the liver to become active or to reach their full potency, so they aren’t active if injected intravenously.
When a drug is taken orally, it must undergo something called “first-pass” metabolism. First-pass metabolism is the metabolism a drug goes through in the liver before it reaches systemic circulation. Usually this reduces the bioavailability of drugs. Alternative routes of administration like suppository, intravenous, intramuscular, inhalational aerosol and sublingual avoid the first-pass effect because they allow drugs to be absorbed directly into the systemic circulation.
When a drug needs to undergo first pass metabolism in order to be active, it is often referred to as a “pro-drug.” A pro-drug has little or no therapeutic effect on its own. It must pass through the metabolism in order to be active. So if a pro-drug is used intravenously, 100% of the pro-drug will be in systemic circulation, but the pro-drug has no therapeutic effect. Pro-drugs are usually designed to improve oral bioavailability of drugs.
Generally though, if the drug is not a pro-drug or similar to a pro-drug, the bioavailability of drugs when administered intravenously is 100%. The second highest bioavailability of drugs is when the drug is administered intramuscularly (75-99%). The third is subcutaneous (75-99 %.)
Following injection, the highest bioavailability of drugs is observed with transdermal administration. Anywhere between 80 and 99% can reach systemic circulation with transdermal administration. Generally, transdermal administration has slow absorption, doesn’t go through first-pass metabolism and prolonged duration of action.
With rectal administration, about half of the drug goes through first-pass metabolism, so the bioavailability of drugs is anywhere between 30 and 99%. Nasal insufflation (snorting) causes a faster onset then other routes of administration, and the bioavailability of drugs is usually (but not always) higher than oral administration. This bioavailability occurs due to the quick absorption of the drug into the bloodstream through the soft tissue in the sinus cavity.
Oral bioavailability of drugs is affected by many different factors. Other medications and food can either decrease or increase the oral bioavailability of drugs. Also, age, gender, medical history, and dose can affect the oral bioavailability of drugs.
Really anything that affects absorbance can affect the bioavailability of drugs. Certain drugs, when they are compounded with other drugs (for instance acetaminophen and oxycodone in Percocet) can affect the absorption with certain routes of administration. For instance, in illicit use, combo drugs like Percocet are often snorted. The acetaminophen can decrease the absorption of oxycodone in the nasal cavity, decreasing the bioavailability. Also, some drugs are specially formulated to prevent abuse. For example, the drug Suboxone is a combination of buprenorphine (an opiate) and naloxone (an opiate blocker). When the drug is used as prescribed (sublingually) the naloxone goes through first pass metabolism and is inactivated. When users try to take the drug intravenously, the naloxone blocks the effect of the buprenorphine.
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