Bathroom medicine cabinets can be bad medicine for drugs.

Toothpaste, shaving creams, aftershave and whatever other nonmedical articles of the toilette you care to indulge in are probably perfectly happy living behind the mirrored door on the bathroom wall.

But medicines-prescription or over-the-counter-tend to lose their potency after a while, especially in less than perfect storage conditions.

Washington pharmacologist Dr. Kenneth Dretcher points out that a drug is "most unstable where it is light , as compared to where it is dark ; most unstable where it is damp as compared to where it is dry ; most unstable where it is hot as compared to where it is cold ."

So what do most of us do? We stick them into our often light, damp and hot bathroom cabinets . . . where they are almost guaranteed to deteriorate.

Better keep them on the dark and dry top shelf of a cabinet in a room you can't go into often, suggests Dretcher, and even then take stock at least once a year. If, as he recommends, you buy only small quantities at a time, replacement expense is minimal, especially considering potential dangers of medicine that is no longer any good.

Well then, what are the medicines that you should have available, even if you shouldn't keep them in the bathroom?

Dretcher agrees with most authorities that ipecac is probably the single most important over-the-counter medicine to have on hand, especially in a house where there are children. Syrup of ipecac is easily obtainable at a pharmacy and its sole function is to induce vomitting. It should be used in certain cases of drug overdoses, especially with children, but it should never be used without first checking with a doctor or a poison-control center, and the victim should be taken to a hospital for further treatment of the overdose. But, says Dretcher, when you do need it, you need it right away. It comes in one-ounce bottles, and only a small amount is generally needed to do its often life-saving, albeit indelicate, thing.

("I used it on one of my children once," he recalled, "although I'm not really certain it was needed . . ." "Oh yes," chimed in a listener, "but it also has a deterrent effect.I once heard my older child say to her younger sister, 'you better not take any medicine yourself. They will give you stuff that makes you throw up . . . and throw up . . . and throw up . . .'")

But beware. When a child has swallowed something that causes chemical burns-such as a caustic cleaner or lye-vomiting can cause more serious damage as the caustic substance comes up, so a doctor or poison-control technician should always be consulted before vomiting is ever induced.

Other handy things to have around the house, says Dretcher:

An analgesic-aspirin or, if aspirin should not be used for some specific reason, one of the aspirin-substitutes such as Tylenol or Datril.

Buy some bottles and even then, suggests Dretcher, open them in the store, take out the cotton and "take a whiff. If the contents smell like vinegar, it means the aspirin has decomposed. Put it back and take another."

An antipruritic-something like calamine lotion-for mosquito bites, poison ivy, poison oak. It soothes itching.

An antiseptic. Nothing better or cheaper than just plain 70 percent isopropyl alcohol, say most authorities. Dretcher suggests it be used for sterilizing utensils and thermometers and not be applied directly on minor wounds, cuts or scrapes. He recommends liberal doses of soap and water for that-and that's all. Some physicians like an antibiotic ointment like Neosporin. Dr. Stanley Talpers, a Washington internist, says he finds it particularly useful for wounds like abrasions-scraped knees, for example-or blisters that break open. "You can't use soap and water to keep a bandage from sticking," Talpers says. Dretcher, who teaches at Georgetown University Medical School, concedes that he has the same argument with his children's pediatrician, but he sticks with soap, mainly because of the cost factor.

A cold preparation. Sudafed or any oral decongestant, Dretcher says, is sufficient, rather than any of the widely-advertised products which contain other things as well-such as antihistamines. Better to take it orally than in the form of nose drops for two reasons: First, you get a greater blood concentration and then, the drops can be irritating or can cause "nasal rebound," a condition more unpleasant than the initial sniffles. (People with diabetes or high blood pressure should not use decongestants at all without a doctor's knowledge, notes Dretcher. They increase blood-sugar levels and raise blood pressure.)

Those are the only things Dretcher feels anyone really needs, but he mentions three secondary items "people will probably have" anyway:

An antacid. If you have one, he warns, know what you are about. Some cause constipation, some diarrhea. People on salt-free diets shouldn't take antacids containing sodium. If you do use one, he notes, liquids are more effective than tablets.

An antidiarrheal. Dretcher finds this unnecessary as diarrhea "is usually self-limiting, gone in two or three days whether it is treated or not," and if it continues, a doctor should be consulted.

A laxative. "I totally object," says Dretcher. "I don't feel they're necessary. There are other ways of combating the problem and besides, we have a hang-up on them."

No other medicines should be kept around at all, he feels, except for specially prescribed drugs for specific conditions. And these should be completely consumed, as prescribed, or thrown away.

Keep in mind that "childproof" tops aren't really "foolproof." Read labels and, when in doubt, call your doctor or your pharmacist. CAPTION: Illustration, no caption