Risk of infection
In some cases, chemotherapy can cause a very low white blood cell count, which can in turn be a risk for infection. If this is the case, it is best to avoid penetrative sex because of the risk of infection. Your doctor may recommend waiting to engage in sexual activity until your white blood cell count has returned to a healthy level.
Tissue thinning
Chemo and radiation can also cause thinning of tissue, which can lead to bleeding during intercourse. If any pain is experienced when engaging in sexual activities during cancer treatment, then sex should be avoided.
Bodily fluid transfer
Bodily fluids can transfer chemotherapy drugs to your partner if you are engaging in penetrative sex, so it is important to use a condom or dental dam so that no fluids are exchanged.