Final answer:
Smoking tobacco increases the metabolism of drugs that are substrates for P450 enzymes like CYP1A2, therefore decreasing the effect of drugs such as warfarin and theophylline.
Step-by-step explanation:
The health impacts of smoking tobacco are extensive and multifaceted, affecting various physiological processes and the metabolism of certain drugs. Specifically regarding the metabolism of drugs, tobacco smoking is known to increase the activity of liver enzymes, particularly the P450 enzymes including CYP1A2. This up-regulation leads to a faster metabolism of drugs that are substrates for these enzymes, thereby often decreasing the effect of these drugs. Warfarin and theophylline are examples of drugs metabolized by CYP1A2, which means smokers may require higher doses to get the same therapeutic effects as non-smokers. In contrast, the absorption of intramuscular or subcutaneously administered drugs could be influenced by changes in blood flow caused by smoking, but specific increases in absorption due to smoking are not well-established.