Hormonal Contraceptives, progestogen-only

Medroxyprogesterone acetate is given in a relatively high dose for hormonal contraception and acts primarily by inhibiting ovulation. However, as with the other progestogen-only contraceptives, other mechanisms probably play a very significant role. It is extremely effective, with less than one pregnancy per 100 woman-years. 

Hormonal contraceptives, which use oral oestrogen and/or progesterone, such as combined hormonal contraceptives and progestogen-only contraceptives. 

Sex hormones and hormone antagonists. Oestrogens increase the synthesis of some vitamin K dependent clotting factors and progestogen-only contraceptives are preferred. The hormone antagonists danazol, flutamide and tamoxifen enhance the effect of warfarin. 

The oral contraceptives are of two main types the combined hormonal contraceptives containing both an oestrogen and a progestogen (monophasic, biphasie, triphasie, or sequential), available as tablets or a patch, and the progestogen-only contraceptives, which are available as tablets (sometimes ealled mini pills), parenteral preparations (implants, depot injections) and intrauterine devices. 

Note that antibacterials that do not induce liver enzymes do not affeet the reliability of the progestogen-only contraceptives, see Progestogen-only contraceptives + Antibacterials, p.l007, or the progestogen-only emergency hormonal contraceptive, see Emergency hormonal contraceptives + Antibacterials , p.977. 

Information is limited, but it appears that these retinoids do not usually alter the efficacy of combined oral contraceptives. The one available case suggests that acitretin reduces the efficacy of progestogen-only oral contraceptives. However, note that progesto n-onty oral contraceptives do not reliably suppress ovulation in all cycles, and that this is not considered their primary mechanism of action (see Hormonal contraceptives and Sex hormones , (p.975)). The single report cannot therefore be taken as evidence that acitretin reduces the efficacy of progestogen-only contraceptives. 

Hormonal contraceptives An interaction of warfarin with hormonal contraceptives has been described in a 33-year-old woman [44 ]. While taking warfarin 38.5 mg/week she switched from a monophasic combined oral contraceptive (ethinylestradiol + nor-ethindrone) to an implantable progestogen-only contraceptive (etonogestrel) 19 days later her INR fell to 1.8 and she required an increase in warfarin dose to 60 mg/week. After 10 months she decided to have the implant removed, and 9 days later her INR rose to 6.5. The warfarin dose was reduced to 55.5 mg/week. She then started to use an oral progestogen-only contraceptive (norethindrone) and the effective warfarin dose was 53.5 mg/ week. The authors hypothesized that the predominant mechanism of this interaction was inhibition of CYP1A2 and CYP2C19 by ethinylestradiol. 

Q10 When choosing an oral contraceptive, a preparation with the lowest oestrogen and progestogen content that can control the cycle is selected. In older women or women with a history of venous thrombosis who require contraception, a progestogen-only preparation may be more suitable than the combined hormonal type, but unfortunately this is not a suitable choice for Shabana s problem. 

The efficacy of both the progestogen-only and combined emergency hormonal contraceptive is likely to be reduced by enzyme inducers such as rifampicin and some antiepileptics. 

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