Which drugs require caution when administered to an older male with bladder flow obstruction due to an enlarged prostate?

Decongestants.

Explanation:

Decongestants are the drugs which are used to relieve nasal congestion and also cause dilation of the blood vessels.

Sympathomimmetics are used as decongestants especially alpha agonists like phenylephrine and pseudoephrine. These drugs stimulate alpha receptors thus increase urinary retention by preventing the muscles in the prostate gland and the bladder neck from relaxing to allow the urine to flow freely and worsen the situation of benign prostatic hyperplasia as bladder flow is already obstructed in this condition.

Hence, the use decongestants is strongly prohibited in the case of urinary bladder obstruction due to and enlarged prostate.