Barbiturates are commonly further classified into different classes by how long they act in the body. As a general rule, the short acting barbiturates are the ones commonly used in anesthesia. These agents start acting quickly but only last a few minutes - which makes them useful for the induction of anesthesia. Thiopental is the prototypical drug used in this way.

Thiopental
The usual does of thiopental is 3-5 mg/kg for induction of general anesthesia. The drug quickly reaches levels in the brain that cause unconsciousness in a matter of seconds. Brain activity is decreased by the drug.

Cardiac depression is a feature of thiopental and it can result in a drop in blood pressure. Healthy patients will respond with a compensatory increase in heart rate in order to maintain their blood pressure - patients that cannot do this due to disease or medication may have a more pronounced drop in blood pressure.

The short duration of action of thiopental is a result of redistribution of the drug from the brain into other tissues. Repeated doses of thiopental can saturate these tissues and result in a long period of unconsciousness rather than the short period that is usually desired.

This drug comes as an alkaline solution with a pH of 10.5 which is water soluble and stable for weeks. While this makes it able to be injected intravenously, it also means that this drug is incompatible in mixture with acidic agents such as opiates and some muscle relaxant medications. Also, the high pH means that injection outside of the intravenous system (into a muscle or under the skin for example), can result in tissue damage. Care must be taken that thiopental is given by itself, and into a freely running intravenous line.

Benzodiazepines
This class of drugs encompasses a large number of medications often given for sedation, anxiety, sleep, etc. Three agents in this class are available for intravenous injection - diazepam (Valium), lorazepam, and midazolam (Versed).

Lorazepam is the least commonly used because it takes as long as twenty minutes for full effect. Valium is commonly used for sedation, reduction of anxiety and amnesia in the period surrounding surgery. It has an onset of action in a few minutes. Versed is very similar to Valium in terms of use and effect - however, it has an even shorter onset of action and the fact that it is water-soluble means that it has less chance of pain on injection.