The Dose and the Timing Make the Poison: Mystery of Thalidomide

pregnancy avoid thalidomideIn many debates over the safety of current chemicals of concern such as bisphenol A (BPA), phthalates and other hormone disruptors, people will make the argument that any chemical, even water, can be toxic because the dose makes the poison. Which is a simple statement of a basic tenet of toxicology. But with some chemicals, namely hormone disruptors (those chemicals that interfere with the endocrine system), many research scientists believe that the statement should be more correctly put as the dose and the timing make the poison. In other words, it isn’t just the amount to which we are exposed, it is when we are exposed – and exposure during certain stages of fetal development is particularly harmful.

The recent announcement of the discovery of mechanism by which thalidomide caused limb defects illustrates this principle. Scientists just solved the 50 year old mystery of how thalidomide caused limb defects.

You may remember thalidomide. Thalidomide was given to women while pregnant to relieve morning sickness. Unfortunately, their babies were born with stunted limbs or sometimes no limbs at all. Thalidomide was identified as the cause, but how it caused these defects was not known. Until just recently.

Scientists have just discovered that a component of the drug prevents the growth of new blood vessels in developing babies, stunting limb growth. As explained by Dr. Neil Vargesson, the lead research, thalidomide was taken around five to nine weeks into a pregnancy when a baby’s limbs are still forming and the blood vessels involved in that process are still growing.  He added, “at this time of the pregnancy the rest of the embryo is unharmed because the blood vessels elsewhere are stable and mature.”

Why am I even talking about this? Because it clearly illustrates that how we determine whether chemicals are safe must take into account how they affect fetuses and children. It isn’t the dose alone – it is the timing. What is safe for an adult may not be safe for a child, even when the dose is scaled for body weight.

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