Final answer:
The fetus is most susceptible to adverse effects from drugs and other teratogens during the embryonic stage, particularly the first eight weeks after fertilization, when major organ development occurs.
Step-by-step explanation:
A fetus is most vulnerable to drug-related adverse effects during the first eight weeks after fertilization, a phase also known as the embryonic stage of development. This is the critical period where all the major organs are forming, and teratogens—substances that can cause developmental disorders—can result in severe structural and functional defects. Alcohol is a well-known teratogen, and exposure during this early stage can cause fetal alcohol spectrum disorders (FASD), which includes a range of lifelong physical, cognitive, and behavioral problems. Other teratogens include illegal drugs like heroin and cocaine, prescription medicines, over-the-counter medications, viruses such as HIV and rubella, and environmental toxins like lead.
In summary, the fetus is most susceptible to teratogens during the first eight weeks of pregnancy. Avoidance of alcohol, drugs, and other harmful substances during this period is crucial for the healthy development of the fetus. Teratogen exposure in this early stage can have irreversible and detrimental effects on fetal development.