In active transport, what happens when multiple drugs compete for the same carrier?

• A. Transport slows unless a drug has higher affinity
• B. Transport increases
• C. Transport is unaffected
• D. Only endogenous substances compete

Answer: (A)

Carrier-mediated active transport is saturable and can be shared by multiple substrates. When two or more drugs try to use the same transporter, they compete for binding to the carrier. This competition effectively reduces the transport rate for each drug at a given concentration because the carrier spends time bound to one substrate and not available to the others. If one drug has higher affinity for the transporter (binds more readily), it will preferentially occupy the carrier and be transported more, leaving less capacity for the other drug. In that situation, the presence of the competing drug slows the transport of the others unless the high-affinity drug dominates the carrier. That's why the correct answer is that transport slows unless a drug has higher affinity.