Capillaries in the glomerulus allow drugs with a MW <60kD to pass into the filtrate. This means most drugs - except large molecules like heparin and protein-bound drugs like warfarin - can be filtered. The amount of filtered drug depends on renal blood flow, glomerular filtration rate and degree of drug binding to plasma proteins.
The glomerular filtration rate varies from individual to individual but in healthy individuals the normal range is 110 to 130 ml/min. About 20% of the blood which enters the glomerulus is filtered. The remaining 80% will pass on to the peritubular capillaries and possibly undergo tubular secretion in the proximal tubules.