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Both enalapril and captopril are medications used in the treatment of high blood pressure caused by hypertension, heart failure, and other medical conditions. These drugs work in similar ways, and are both classified as angiotensin-converting enzyme (ACE) inhibitors due to their action of preventing the ACE protein from constricting the blood vessels of the body. There are some differences between these medications, despite their similarities, that can lead to doctors choosing to prescribe one over the other in certain situations.
One major difference between enalapril and captopril pertains to how they are administered to patients. Enalapril may be given to patients as a tablet to take orally, or it may be injected intravenously (IV) for immediate action. Captopril, on the other hand, is always given to the patient orally, meaning that it is not typically used to relieve emergency hypertensive crises.
The difference in the way that enalapril and captopril are administered has to do with their relative strengths. Dosages of captopril are higher, starting at 25 milligrams (mg), because it is considered a weaker drug. Additionally, it has a higher oral bioavailability than enalapril, meaning that it absorbs into the bloodstream more effectively when taken by mouth. Initial active doses for enalapril are 5 mg for oral use, and 1.25 mg when given in IV form, due to its more efficient means of blocking the ACE protein.
Duration of action is another point of disparity between enalapril and captopril. Enalapril has a 19 hour half-life, or the time it takes for half of the drug to be broken down and removed from the body after ingestion. The half-life of captopril is only 1.9 hours, which is another reason it is not used in hypertensive emergencies. Shorter half-lives require additional dosing at more frequent intervals, which is undesirable during a serious medical situation.
The side effects of enalapril and captopril show some variation. ACE inhibitors, including both of these drugs, tend to cause dizziness, hypotension, and coughing. Additional side effects can be seen with captopril that are not present in other drugs of its class, however. It contains a sulfur atom in its structure that other ACE inhibitors do not have, leading to unique potential side effects including rashes, and a lingering metallic taste on the tongue that some patients experience. Postural hypotension, or a loss of blood pressure when rapidly standing up, is another side effect occurring only in captopril, and exists because of the drug's rapid onset speed.