What effect does molecular weight less than 200 typically have on drug transfer across membranes?

A molecular weight of less than 200 daltons generally indicates that a substance is small enough to effectively diffuse across biological membranes, which are primarily lipid bilayers. This size allows for enhanced transfer and easier passage of the drug through membranes. Small molecules can more readily navigate through the lipid composition of the cell membrane using passive diffusion, facilitated diffusion, or even active transport if they have certain transport proteins associated with them.

The ability for smaller molecules to traverse membranes is crucial for effective drug delivery, as it allows them to reach their target sites within the body more efficiently. When a drug has a low molecular weight, it typically has a higher permeability, which can lead to increased bioavailability and a more rapid onset of action.

In contrast, larger molecules often face barriers due to size, making it difficult for them to cross membranes. This highlights the significant role that molecular weight plays in pharmacokinetics, particularly in relation to the absorption and distribution phases of drug action.