An oral drug – better known to the public by its proprietary names Viagra® and Relation® – for treating erectile dysfunction. It is one of the VASODILATORS, so enhances blood flow to the penis. It works in response to sexual stimulation and has no properties as an aphrodisiac nor does it provoke sexual fantasies. There are restrictions on how NHS doctors may prescribe these drugs. Side-effects included headache, facial flushing, indigestion and a stuffy nose. Sildenafil must not be taken with drugs containing nitrates such as GLYCERYL TRINITRATE or isosorbide trinitrate as the subject may suffer a sudden fall in blood pressure. Nitrates inhaled for recreational use (poppers) have a similar effect (see also ERECTION; SEXUAL DYSFUNCTION).