What is the difference between the anti-allergic drugs cetirizine and levocetirizine, as there is an extra "left" in their names?

Cetirizine is a commonly used drug for the treatment of allergic diseases. But many people asked Huazi, there is another drug called levocetirizine, which only has one more "left" than the former, what does it mean, how big is the difference between the two, and if there is levocetirizine, should there also be dextrocetirizine?

Hua Zi said that in essence, the two belong to the same drug. The active ingredient in cetirizine is levocetirizine. The two have the same ingredients to exert their efficacy, but the side effects of levocetirizine are less than those of cetirizine.

1. Magical isomers Cetirizine is actually composed of two substances, levocetirizine and dextrocetirizine. The "left" and "right" here refer to the arrangement direction of the drug molecules in space. In other words, although the molecules that make up the two are exactly the same, their directions in space are different.

The simplest example is like our left and right hands, which have the same structure, five fingers and a palm. But the left and right hands are in different directions in space, showing mirror symmetry. The same is true for the drug molecules of cetirizine. If polarized light is used to detect, half of the drug has an optical rotation to the left, and the other half has an optical rotation to the right. Cetirizine is a mixture of the two, also known as racemic cetirizine.

2. Differences in the properties of isomers Although the drug molecules of levocetirizine and dextrocetirizine are exactly the same, except for the different spatial orientations, their properties are very different. Levocetirizine has a strong blocking effect on peripheral H1 receptors and is the main component that exerts anti-allergic effects. Dextrocetirizine has a weak anti-allergic effect, but has a certain affinity with receptors in the brain, which can cause side effects such as sedation and drowsiness.

Therefore, people purified the levocetirizine from cetirizine and discarded the right cetirizine, which not only retained the anti-allergic effect of the original drug, but also reduced the adverse reactions of the drug. Therefore, the daily dosage of cetirizine is 10 mg, while the daily dosage of levocetirizine is 5 mg, because half of the right cetirizine is discarded.

3. Levocetirizine is a prescription drug. In terms of classification, cetirizine is an over-the-counter (OTC) drug that can be purchased and used by yourself. However, levocetirizine is a prescription drug that requires a doctor's prescription to purchase and use. This is because cetirizine was launched earlier, more clinical trials have been conducted, and the research is more thorough. Although the side effects are greater, the safety is better. Levocetirizine is relatively new, and it will take longer to verify its safety.

Although cetirizine has slightly greater side effects, in actual use, some patients who are ineffective with levocetirizine will still get better results by switching to cetirizine. Therefore, cetirizine is still commonly used. Both are excreted through the kidneys, and those with decreased renal function should adjust the dose, and those with severe renal insufficiency should not use it.

Levocetirizine has milder side effects, but adverse reactions such as drowsiness, fatigue, and headache are still common. Therefore, it is recommended that both cetirizine and levocetirizine be taken orally before bedtime to reduce the incidence of adverse reactions. The plasma elimination half-life of both is about 10 hours, so it can be taken once a day.

People who take cetirizine should not drive vehicles, operate machinery, or perform other dangerous activities such as working at heights during medication. People who take levocetirizine generally do not have their alertness affected at normal doses. If you need to engage in dangerous activities, be careful not to overdose.

To sum up, the main difference between cetirizine and levocetirizine is the side effects. The side effects of levocetirizine are significantly lower than those of cetirizine, and the efficacy of cetirizine is actually slightly higher than that of levocetirizine. When using it, it is recommended to use the medicine under the guidance of a doctor. If you have any questions about the use of the medicine, please consult a doctor or pharmacist in time. I am pharmacist Huazi, welcome to follow me and share more health knowledge.