Interaction With Other Drugs
Dr. Álvaro Monterrosa Castro, MD
TOSome drugs reduce the effectiveness of oral contraceptives (7). And oral contraceptives can increase or decrease the effects of other preparations. (twenty).
First of all:
Anticonvulsants: Carbamazepine, Primidone, Phenobarbital and Phenytoin. They induce the synthesis of Cytochrome P-450 enzymes in the Liver. Accelerating the metabolism of steroid hormones. And therefore reduce plasma Ethinyl-estradiol levels. Lowering contraceptive effectiveness (7,207,208, 209).
In second place;
There are publications that established an increase in pregnancy rates in oral contraceptive users who received antibiotics. (208,209).
First, Rifampin decreases contraceptive efficacy because it is a potent inducer of Cytochrome P-450. Which reduces blood levels of Ethinyl-estradiol and Norethindrone (twenty).
(Read Also: Oral Contraception Counseling)
Second, Ampicillin and Tetracycline reduce the bacterial flora of the digestive tract, especially Clostridium. Responsible for the hydrolysis of steroid glucomates in the intestine. Event that allows reabsorption of the steroid by the enterohepatic circulation (twenty).
Likewise, Csemiczky et al (209) evaluated the possibility of ovulation escape in users of oral contraceptives (150 ud of Levonorgestrel + 30 ug of Ethinyl-Estradiol) during treatment with ofloxacin. The authors conclude that no additional contraceptive precautions are needed when users receive this Quinolone.
In addition, oral contraceptive users who ingest Ascorbic Acid, Acetaminophen. They present an increase in plasma Ethinyl-estradiol levels (20).
Although recently Huhnz et al (210) They assert that Vitamin C does not cause an interaction with the pharmacokinetics of the pill. Oral contraceptives reduce metabolic clearance and increase the half-life of Benzodiazepines, Imipramine, Prednisone(7).
In turn, they decrease the clearance of Caffeine, Theophylline and Ethanol. While they increase the clearance of Acetyl-Salicylic Acid and Morphine. As a consequence, their plasma levels are lowered, requiring higher doses for an adequate therapeutic effect. (twenty).
Dr. Álvaro Monterrosa Castro, MD
