Vitamin C Interaction with Ethinyl Estradiol

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Questions and Answers

What is the effect of ascorbic acid on ethinyl estradiol plasma level when taken concurrently?

Decreases the plasma level

Why does the plasma level of ascorbic acid decrease when taken with ethinyl estradiol?

Due to the release of ceruloplasmin with ascorbate oxidase activity

What is the recommendation for patients taking vitamin C and oral contraceptives containing estrogens?

Warn patients about possible failure of contraception

What is the effect of antacids on the oral bioavailability of ferrous salts?

Decreases the oral bioavailability Signup and view all the answers

Why does the presence of antacids decrease the oral bioavailability of ferrous salts?

Due to decreased solubilization of ferrous salts at higher pH Signup and view all the answers

What is the recommendation for administering ferrous salts with antacids?

Administer them separately with a long interval Signup and view all the answers

What is the effect of theophylline on erythromycin plasma levels?

Decreases the plasma level Signup and view all the answers

What is the expected interaction between other xanthines and erythromycin?

The same interaction as theophylline and erythromycin Signup and view all the answers

What is the effect of propranolol on theophylline plasma levels?

Increases the plasma level Signup and view all the answers

Why does propranolol pharmacologically antagonize the effect of theophylline on bronchial musculature?

Propranolol increases bronchial resistance Signup and view all the answers

What is the expected interaction between dyphylline and propranolol?

Pharmacological antagonism but no effect on dyphylline plasma level Signup and view all the answers

What is the effect of ascorbic acid on ethinyl estradiol oral bioavailability?

Decreases the oral bioavailability Signup and view all the answers

What is recommended when administering theophylline with erythromycin?

Monitoring of plasma levels of both theophylline and erythromycin Signup and view all the answers

Which beta-blockers do not affect the pharmacokinetics of theophylline?

Atenolol and bisoprolol Signup and view all the answers

What is the expected outcome of concurrent administration of theophylline and erythromycin?

A 2-fold increase in the plasma level of theophylline Signup and view all the answers

Why does dyphylline not interact with thiabendazole?

Because it is not converted to theophylline Signup and view all the answers

What is the mechanism of the interaction between thiabendazole and theophylline?

Thiabendazole inhibits the metabolism of theophylline Signup and view all the answers

What is the recommended course of action for patients receiving xanthine derivatives and thiabendazole?

Close monitoring of theophylline plasma level and potential dose reduction Signup and view all the answers

Which of the following xanthine derivatives is expected to interact with halothane?

All xanthine derivatives, except dyphylline Signup and view all the answers

What is the recommended course of action for asthmatic patients receiving xanthine derivatives and requiring anesthesia?

Use an anesthetic agent other than halothane Signup and view all the answers

Which of the following antibiotics does not interact with theophylline?

Albendazole Signup and view all the answers

What is the effect of erythromycin on the plasma level of theophylline?

It increases the plasma level of theophylline Signup and view all the answers

Study Notes

Vitamin C and Ethinyl Estradiol Interaction

Vitamin C (ascorbic acid) competes with sulfate in the gut wall, leading to enhanced absorption of vitamin C.
Concurrent administration of vitamin C and ethinyl estradiol results in decreased plasma levels of ethinyl estradiol.
If vitamin C is withdrawn, there is a sudden increase in ethinyl estradiol plasma levels, leading to breakthrough bleeding and failure of contraception.
The decrease in plasma ascorbic acid is attributed to the release of hepatic ceruloplasmin, which has ascorbate oxidase activity.

Ferrous Salts and Antacids Interaction

Concurrent administration of ferrous salts and antacids results in decreased oral bioavailability of iron.
This interaction is due to the fact that dissolution and solubilization of ferrous salts require acidic pH.
At higher pH, ferrous salts solubilization is markedly decreased, leading to formation of clusters and decreased absorption of iron.

Theophylline and Erythromycin Interaction

Concurrent administration of theophylline and erythromycin results in decreased plasma levels of erythromycin.
Theophylline reduces the plasma level of erythromycin, but the exact mechanism is not known.

Theophylline and Propranolol Interaction

Concurrent administration of theophylline and propranolol results in increased plasma levels of theophylline.
Propranolol pharmacologically antagonizes the effect of theophylline on bronchial musculature.
This interaction is attributed to the inhibition of phosphodiesterase enzyme by theophylline and the increase in bronchial resistance by propranolol.

Ascorbic Acid and Ethinyl Estradiol Interaction

Concurrent administration of ascorbic acid and ethinyl estradiol results in increased oral bioavailability of ethinyl estradiol.
This interaction is due to the release of hepatic ceruloplasmin, which has ascorbate oxidase activity.

Theophylline and Halothane Interaction

Concurrent administration of theophylline and halothane results in increased pharmacological effect of theophylline.
This interaction is due to the inhibition of theophylline metabolism by halothane.

Aminophylline and Thiabendazole Interaction

Concurrent administration of aminophylline and thiabendazole results in increased plasma levels of theophylline.
This interaction is attributed to the inhibition of liver microsomal enzyme by thiabendazole, leading to decreased metabolism of theophylline.

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