Which routes can drugs be administered and how is one chosen? i-checksum="38458" ends

Oral administration

The most frequently employed route for giving medication is by mouth. The drug is usually absorbed through the lining of the small bowel after passage through the stomach. The rate of gastric (stomach) emptying varies between individuals and this may explain some differences in drug response. Absorption of hormones may be reduced by chronic bowel conditions, such as coeliac disease, or an acute gastrointestinal illness involving vomiting and diarrhoea. This may result in unwanted pregnancy in women taking oral contraceptives, or irregular bleeding in women taking either the oral contraceptive pill or HRT. The circulation is such that blood from the small bowel travels directly to the liver. The liver may break down most of the drug on this ?first pass?.

Other routes are chosen for a variety of reasons including problems getting the drug safely through the acid milieu of the stomach and yet in a suitable state for absorption by the small bowel and also if the liver will let very little through during the first pass after absorption from the bowel. 

Vaginal or rectal routes

Rectal or vaginal progesterone suppositories/pessaries have been frequently prescribed for patients with premenstrual syndrome (PMSprogesterone and progestogen in PMS), for infertility including IVF (Q10.24) and to try to reduce miscarriages for those with recurring problems (Q12.16). More recently a vaginal progesterone gel (Crinone Serono) has been marketed for infertility, PMS and in the management of the menopause. The vagina has been shown to be a useful route of administration of oestrogen in hormone replacement therapy (Menoring estradiol acetate Galen Ltd).

Some drugs that are usually taken by mouth, such as bromocriptine and cabergoline (Q6.21) may cause nausea or vomiting even when taken after meals, as they have a direct action on the stomach. The vagina may prove to be a useful alternative route.

Intramuscular and subcutaneous routes

Most drugs are water-soluble and they are readily absorbed by the small intestine, or they are lipid (fat) soluble. Lipid soluble drugs are absorbed when injected into a muscle (intramuscular) or into the fat under the skin (subcutaneous). A depot preparation of medroxyprogesterone acetate provides reliable contraception for three months (Depo-Provera). Diabetics have undertaken self-injection of insulin for many years. Patients are increasingly being taught to self-inject hormones for infertility treatment.

Implants of oestradiol and testosterone are inserted under the skin for some women requiring hormone replacement therapy (Q 28.5). Similarly, one GnRH agonist implant, goserelin (Zoladex -Zeneca) is introduced monthly for some patients with endometriosis (endometriosis) or with fibroids before surgery (17).

Transdermal route

There are a variety of patches favoured by some patients for their hormone replacement. Some contain oestradiol only (Table 28.2) while others have oestradiol combined with progestogen either cyclically or continuously.

Nasal route

Gonadotrophin-releasing hormone agonists, including nafarelin and buserelin, can be administered by nasal spray for treatment of endometriosis, pre-operative preparation for myomectomy (fibroid removal -17), preparation of the endometrium before endometrial ablation (endometrial ablation) and in some infertility regimens (Q10.24). A new HRT preparation (Aerodiol estradiol hemihydrate - Servier) is administered by nasal spray (Q 28.5).

Intrauterine route

The levonorgestrel intrauterine system (Mirena) is an intrauterine contraceptive device (IUCD) that releases the e quivalent of two progestogen only pills each week. It is licensed for contraception (Mirena) but it may also have a place in the management of dysfunctional uterine bleeding (heavy periods with no specific causeQ14.36). It provides not only contraception in pre-menopausal women but endometrial protection when oestrogen is administered either to control premenstrual (Q14.37) or menopausal symptoms (Q 28.15). A variety of other applications are under consideration including the management of endometriosis.