istration of oral combination hormonal contraceptives; significant changes (increase and decrease) in the plasma levels of the estrogen and progestin have been noted in some cases. The safety and efficacy of oral contraceptive products may be affected with coadministration of anti-HIV protease inhibitors. Health care professionals should refer to the label of the individual anti-HIV protease inhibitors for further drug-drug interaction information.
Herbal products containing St. John's Wort (Hypericum perforatum) may induce hepatic enzymes (cytochrome P 450) and p-glycoprotein transporter and may reduce the effectiveness of contraceptive steroids. This may also result in unscheduled bleeding.
Increase in Plasma Levels Associated with Coadministered Drugs:
Coadministration of atorvastatin and certain oral contraceptives containing ethinyl estradiol increases AUC values for ethinyl estradiol by approximately 20%. Ascorbic acid and acetaminophen increase the bioavailability of ethinyl estradiol since these drugs act as competitive inhibitors for sulfation of ethinyl estradiol in the gastrointestinal wall, a known pathway of elimination for ethinyl estradiol. CYP 3A4 inhibitors such as indinavir, itraconazole, ketoconazole, fluconazole, and troleandomycin may increase plasma hormone levels. Troleandomycin may also increase the risk of intrahepatic cholestasis during coadministration with combination oral contraceptives.
Changes in Plasma Levels of Coadministered Drugs:
Combination hormonal contraceptives containing some synthetic estrogens (eg, ethinyl estradiol) may inhibit the metabolism of other compounds. Increased plasma concentrations of cyclosporine, prednisolone and other corticosteroids, and theophylline have been reported with concomitant administration of oral contraceptives. Decreased plasma concentrations of acetaminophen and lamotrigine, and increased clearance of temazepam, salicylic acid, morphine, and clofibric acid, due to induction of conjugation (particularly glucuronidation), have been noted when these drugs were administered with oral contraceptives.
The prescribing information of concomitant medications should be consulted to identify potential interactions.
10. Interactions with Laboratory Tests
Certain endocrine- and liver-function tests and blood components may be affected by oral contraceptives:
Increased prothrombin and factors VII, VIII, IX, and X; decreased antithrombin 3; increased norepinephrine-induced platelet aggregability.
Increased thyroid-binding globulin (TBG) leading to increased circulating total thyroid hormone, as measured by protein-bound iodine (PBI), T4 by column or by radioimmunoassay. Free T3 resin uptake is decreased, reflecting the elevated TBG; free T4 concentration is unaltered.
Other binding proteins may be elevated in serum ie, corticosteroid binding globulin (CBG), sex hormone-binding globulins (SHBG) leading to increased levels of total circulating corticosteroids and sex steroids, respectively. Free or biologically active hormone concentrations are unchanged.
Triglycerides may be increased and levels of various other lipids and lipoproteins may be affected.
Glucose tolerance may be decreased.
Serum folate levels may be depressed by oral contraceptive therapy. This may be of clinical significance if a woman becomes pregnant shortly after discontinuing ora
