What are the factors influencing drug bioavailability?

First pass liver metabolism (nitroglycerin has 10% bioavailability due to first pass) Solubility of drug (Hydrophilic products are poorly absorbed, extremely lipophillic are also poorly absorbed since they cannot circulate in the blood - this is why drugs are either weak acids or bases) Chemical Instability (Degrarded by enzymes (insulin) and acidity (penicillin)) Nature of drug Formulation (size, shape, crystrallization, coating)