A patient calls asking you if the 'prescription pills' they got about 8 months ago are still good? The prescription is for an antihistamine they only use occasionally when symptoms occur during the spring. They claim it's been stored at room temp since they received it.

What do you tell them?

I was taught in pharmacy school a general rule of thumb - prescription drugs when stored properly are good for a year after being dispensed. The key is being stored properly - not in a steamy bathroom for a year or in the glove compartment of you car or in an envelope because the bottle got lost. Again, this is just a general rule of thumb. If you try to ask a manufacturer about use beyond the exp date, they'll say NO. Because of legal and liability concerns, the drug manufacturer will not endorse such use and may not even comment on the safety and efficacy of such practice.

A little more scientific info.... The expiration date is usually a mathematical extrapolation of degradation of the drug. The manufacturer studies the drug under set condition and periodically measures potency. From this data, they extrapolate when the expiration date should be. The expiration date is the calculated date when a drug's potency, stored in a sealed original container under controlled conditions, falls below the minimum standards set by the US Pharmacopoeia (USP) or National Formulary (NF). This threshold is usually 90 to 95% of the labeled potency, depending on the drug. For example, narrow therapeutic index drugs (warfarin, digoxin, etc) will have a tighter range. For example, drug "x" 100mg tablets expire when the drug potency of these tablets reaches 90 or 95 mg, depending on the USP or NF standards. This applies to solid oral dosage forms (tablets, capsules).

Liquid dosage forms are generally not as stable as solid dosage forms. Suspensions are especially susceptible to freezing as the suspension may 'break' and separate into layers. Once these suspensions 'break' they will not so back into suspension. Drugs in solution showing signs of discoloration or recipitation shouldn't be use. Extremes in temperature, humidity, or exposure to light accelerate the degradation process. With some liquid dosage forms, the potency of the drug MAY be retained but the preservative's ability to inhibit microbial growth may be impaired.

Can drugs break down into some toxic compound? There are reports of tetracycline breaking down into a 'toxic' compound causing renal toxicity (Fanconi's syndrome). The Medical Letter (2002;44:93-4) states, "There are virtually no reports of toxicity from degradation products of outdated drugs." Whether they don't cause toxicity or the incidences just haven't been reported cannot be determined.