Tubular Secretion: affected by drug competition (penicillins)
Drug-Drug Interactions: Pharmacodynamics
Interacting at same receptor: inhibitory (narcan for morphine)
Interacting at different sites: potentiating, inhibitory, or neutral: 2 drugs with sedative properties, etc.
Drug-Drug Interactions: Toxicity
additive or synergistic
Examples: two sedative drugs, two drugs with kidney toxicity (aminoglycoside antibiotic+vanomycin), two drugs with liver toxicity (TB drugs: INH+PZA), two or more drugs that cause potassium (corticosteroids+thiazide diuretics).