Propofol is a synthetic compound called 2,6-diisopropylphenol, with the molecular formula C₁₂H₁₈O. It belongs to the phenol family, meaning its core structure is a ring of carbon atoms with a hydroxyl (oxygen-hydrogen) group attached. Two bulky isopropyl groups sit on either side of that hydroxyl group, giving the molecule the shape and oil-soluble properties that define how it works and how it has to be delivered.
But the active molecule is only part of the story. Because propofol doesn’t dissolve in water, it can’t simply be mixed into a saline solution and injected. Instead, it’s suspended in a milky white fat-based emulsion that looks remarkably like milk, and each ingredient in that emulsion serves a specific purpose.
The Active Ingredient
Propofol itself is a small, oily liquid at room temperature. Its chemical job is straightforward: it enhances the activity of the brain’s main “calm down” signaling system. Specifically, it latches onto receptors that respond to a neurotransmitter called GABA, acting as a booster that amplifies GABA’s natural inhibitory signal. This pushes the brain into unconsciousness within about 30 to 60 seconds of injection. The molecule binds at multiple sites within the spaces between the receptor’s protein subunits, which is why it produces such a rapid, deep effect.
The Lipid Emulsion
Since propofol won’t dissolve in water on its own, it’s delivered as an oil-in-water emulsion, similar in concept to the fat emulsions used in intravenous nutrition. The brand-name version, Diprivan, lists these inactive ingredients per milliliter:
- Soybean oil (100 mg/mL): The oil phase that actually dissolves the propofol. It forms tiny droplets, each about 150 to 200 nanometers across, suspended throughout the liquid.
- Egg lecithin (12 mg/mL): A phospholipid extracted from egg yolks that acts as the emulsifier. It coats each oil droplet to keep it from merging with its neighbors, maintaining the smooth, stable suspension.
- Glycerol (22.5 mg/mL): Adjusts the osmolality of the emulsion so it matches the concentration of your blood, preventing damage to veins and red blood cells during injection.
- Sodium hydroxide: Added in small amounts to bring the pH into the target range. Diprivan sits at a pH of roughly 7 to 8.5.
The result is a white, opaque liquid that is often compared to milk. That milky appearance comes directly from light scattering off billions of tiny soybean oil droplets.
Antimicrobial Additives
The fat-rich emulsion is an appealing environment for bacteria, which creates a contamination risk once a vial is opened. Manufacturers address this differently depending on the brand. Diprivan includes disodium edetate (EDTA) at 0.05 mg/mL as its antimicrobial agent and keeps its pH near neutral. Some generic formulations use sodium metabisulfite instead, paired with a lower pH of around 4.5 to 6.4 to further inhibit microbial growth. That distinction matters for the small number of patients with sulfite sensitivities, since metabisulfite-containing versions could trigger a reaction in those individuals.
Egg and Soy Allergies
The presence of egg lecithin and soybean oil naturally raises concern for people with egg or soy allergies. In practice, the risk is far lower than most people assume. Food allergies to egg and soy are driven by proteins in those foods, not by their fats or phospholipids. While soybean oil and egg lecithin can contain trace residual protein, no allergic reactions have been conclusively linked to those traces. The American Academy of Allergy, Asthma & Immunology states that patients with soy or egg allergies can receive propofol without special precautions. Most reported cases of anaphylaxis to propofol have occurred in people with no egg allergy at all, suggesting the rare allergic reactions to propofol are triggered by the propofol molecule itself or some other mechanism that remains unclear.
The Water-Soluble Alternative
The lipid emulsion format has real drawbacks. It can raise triglyceride levels during prolonged use, increases the risk of bacterial contamination, and in rare cases contributes to inflammation of the pancreas with long-term infusion. To get around these issues, a prodrug called fospropofol disodium was developed. Chemically, it’s propofol with a phosphate group attached, which makes it fully water-soluble and eliminates the need for an oil emulsion entirely.
Once injected, enzymes in the blood strip away the phosphate group and release active propofol along with phosphate and formaldehyde (the formaldehyde is quickly converted to formate and cleared). The tradeoff is speed: fospropofol takes 4 to 8 minutes to reach its effect compared to propofol’s 30 to 60 seconds, and it produces lower peak drug levels with a more gradual onset. That slower profile can actually be an advantage in critically ill patients who need stable blood pressure, but it makes fospropofol a poor choice when rapid sedation is the goal.