Dose Dose by definition is the amount of a substance administered at one time. However, other parameters are needed to characterize the exposure to xenobiotics.  The most important are the number of doses, frequency, and total time period of the treatment. For example: 650 mg Tylenol as a single dose 500 mg Penicillin every 8 hours for 10 days 10 mg DDT per day for 90 days There are numerous types of doses, e.g., exposure dose, absorbed dose, administered dose and total dose. Fractionating a total dose usually decreases the probability that the total dose will cause toxicity.  The reason for this is that the body often can repair the effect of each subtoxic dose if sufficient time passes before receiving the next dose.  In such a case, the total dose, harmful if received all at once, is non-toxic when administered over a period of time.  For example, 30 mg of strychnine swallowed at one time could be fatal to an adult whereas 3 mg of strychnine swallowed each day for ten days would not be fatal. The units used in toxicology are basically the same as those used in medicine.  The gram is the standard unit.  However, most exposures will be smaller quantities and thus the milligram (mg) is commonly used.  For example, the common adult dose of Tylenol is 650 milligrams. The clinical and toxic effects of a dose must be related to age and body size. For example, 650 mg is the adult dose of Tylenol.  That would be quite toxic to young children, and thus Children's Tylenol tablets contain only 80 mg.  A better means to allow for comparison of effectiveness and toxicity is the amount of a substance administered on a body weight basis.  A common dose measurement is mg/kg which stands for mg of substance per kg of body weight. Another important aspect is the time over which the dose is administered.  This is especially important for exposures of several days or for chronic exposures.  The commonly used time unit is one day and thus, the usual dosage unit is mg/kg/day. Since some xenobiotics are toxic in much smaller quantities than the milligram, smaller fractions of the gram are used, such as microgram (µg).  Other units are shown below: Environmental exposure units are expressed as the amount of a xenobiotic in a unit of the media. mg/liter (mg/l) for liquids mg/gram (mg/g) for solids mg/cubic meter (mg/m3) for air Smaller units are used as needed, e.g., µg/ml.  Other commonly used dose units for substances in media are parts per million (ppm), parts per billion (ppb) and parts per trillion (ppt).