Appendix 6b:

Drug–Contraceptive Interactions

CONTRACEPTIVES, ORAL

Acetazolamide

Antagonism of diuretic effect

Amiloride

Antagonism of diuretic effect

Amitriptyline

Antagonism of antidepressant effect but adverse effects increased due to increased plasma concentration of amitriptyline

Amoxycillin

Reduced contraceptive effect of estrogen-containing preparations

Ampicillin

Reduced contraceptive effect of estrogen-containing preparations

Atenolol

Antagonism of hypotensive effect

Carbamazepine

Accelerated metabolism of medroxyprogesterone (does not apply to injectable medroxyprogesterone acetate for contraception); reduced contraceptive effect (does not apply to injectable norethisterone enantate for contraception)

Ceftazidime

Reduced contraceptive effect of estrogen-containing preparations

Ceftriaxone

Reduced contraceptive effect of estrogen-containing preparations

Cefuroxime

Reduced effect of contraceptives

Corticosteroids

Oral contraceptives increase plasma concentration of corticosteroids

Clomipramine

Antagonism of antidepressant effect but adverse effects increased due to increased plasma concentration of clomipramine

Cyclosporine

Inhibition of cyclosporine metabolism (increased plasma–cyclosporine concentration)

Doxycycline

Reduced contraceptive effect of estrogen-containing preparations

Efavirenz

Efficacy of oral contraceptives reduced

Enalapril

Antagonism of hypotensive effect

Fluconazole

Anecdotal reports of contraceptive failure

Fosphenytoin

Reduced contraceptive effect

Furosemide

Antagonism of diuretic effect

Glibenclamide

Antagonism of hypoglycaemic effect

Glimepiride

Reduced hypoglycaemic action

Glucagon

Antagonism of hypotensive effect

Glyceryl trinitrate

Antagonism of hypotensive effect

Griseofulvin

Reduced contraceptive effect of levonorgestrel, accelerated metabolism of medroxyprogesterone (does not apply to injectable medroxyprogesterone acetate for contraception); does not apply to injectable norethisterone enantate for contraception

Hydralazine

Antagonism of hypotensive effect

Hydrochlorothiazide

Antagonism of diuretic effect

Insulins

Antagonism of hypoglycaemic effect

Isosorbide dinitrate

Antagonism of hypotensive effect

Metformin

Antagonism of hypoglycaemic effect

Methyldopa

Antagonism of hypotensive effect

Nelfinavir

Accelerated metabolism of levonorgestrel, medroxyprogesterone and norethisterone (reduced contraceptive effect); does not apply to injectable medroxyprogestrone acetate for contraception

Nevirapine

Accelerated metabolism of levonorgestrel, medroxyprogesterone and norethisterone (reduced contraceptive effect); does not apply to injectable medroxyprogestrone acetate for contraception

Nifedipine

Antagonism of hypotensive effect

Phenobarbital

Metabolism accelerated (reduced contraceptive effect); does not apply to injectable medroxyprogestrone acetate for contraception; does not apply to injectable norethisterone enantate for contraception

Phenytoin

Accelerated metabolism of levonorgestrel, norethisterone (reduced contraceptive effect); does not apply to injectable medroxyprogestrone acetate for contraception; does not apply to injectable norethisterone enantate for contraception

Propranolol

Antagonism of hypotensive effect

Rifampicin

Accelerated metabolism of levonorgestrol and medroxyprogesterone (reduced contraceptive effect); does not apply to injectable medroxyprogestrone acetate for contraception; does not apply to injectable norethisterone enantate for contraception

Ritonavir

Accelerated metabolism of levonorgestrol and norethisterone (reduced contraceptive effect); does not apply to injectable medroxyprogestrone acetate for contraception; does not apply to injectable norethisterone enantate for contraception

Sodium nitroprusside

Antagonism of hypotensive effect

Spironolactone

Antagonism of diuretic effect

Topiramate

Failure of contraceptive effect

Theophylline

Delayed excretion of theophylline; increased plasma concentration

Verapamil

Antagonism of hypotensive effect

Warfarin

Antagonism of anticoagulant effect