What is the primary route of drug introduction that takes the longest time to take effect?

Ingestion is the primary route of drug introduction that typically takes the longest time to take effect. When a substance is ingested, it has to undergo several processes before it can exert its effects. First, the drug is swallowed and enters the digestive system, where it must be broken down by stomach acids and enzymes. After this, it is absorbed through the intestinal walls into the bloodstream, which can take time as compared to other routes like inhalation or injection.

Inhalation delivers substances directly to the bloodstream through the lungs, resulting in almost immediate effects. Similarly, injection facilitates rapid absorption by directly introducing the drug into the circulation. Absorption, particularly through skin or mucous membranes, can also take effect relatively quickly but is usually not as immediate as injection or inhalation. Due to the complex processes involved with ingestion, such as digestion and absorption, this route generally has a delayed onset of effects compared to the others.