Five years ago, sex became "very stressful" for Mr Tan because of his inability to maintain an erection.
The 43-year-old sales executive, who did not want to give his real name, said: "I did not know if sex that night was going to be satisfactory or if I would be a party pooper."
Mr Tan, who has a girlfriend, saw a general practitioner and found relief in "yellow and blue pills" that he popped an hour before sex.
Well-known by their trade names, Viagra, Cialis or Levitra, these pills belong to a class of medication known as phosphodiesterase-5 (PDE-5) inhibitors, which block the PDE-5 enzyme found in blood vessel walls. They help the smooth muscles of the penis to relax and increase blood flow to the organ, but are also associated with side effects such as headaches and flushing.
But two years ago, the pills stopped working after Mr Tan had an angioplasty procedure to unblock an artery, so he turned to second-line treatments - penile injection therapy and mechanical aids.
He would inject small amounts of medicine into the side of his genitals several minutes before sex to boost blood flow into the organ.
Or he would use a vacuum device to draw blood into the penis, then slip a constriction ring onto its base to maintain the erection.
While these worked for him, he felt that they were cumbersome and took away the spontaneity of sex.
"We always tried to plan for sex and, even then, I always had doubts about my erections."
Mr Tan was also offered a penile prosthesis - an implant that facilitates an erection - but declined the option as it involves surgery.
In June, he was offered fresh hope by senior consultant urologist Peter Lim, who runs a practice at Gleneagles Hospital.
Twice a week, for three weeks, Mr Tan had low-intensity shock waves delivered to five areas of his penis to stimulate new blood vessels to grow. The treatment can be repeated if necessary.
The exact mechanism of the low-intensity extracorporeal shock wave therapy remains unclear, but the aim is to restore the erectile mechanism for natural or spontaneous erections.
It is the latest advancement in erectile dysfunction (ED) treatment since pharmaceutical giant Pfizer launched Viagra in 1998.
The outpatient procedure does not require any downtime or anaesthesia.
Mr Tan returned to work after each 20-minute session, which left him with only a tingling sensation.
He said his sex life has been "good so far" and is close to what he was able to achieve in his 20s.
These days, he has sex twice a week, but hopes to "rev this up to six times a week", he declared.
Sending shock waves
Shock waves are acoustic waves that carry energy and can be used to affect a selected anatomical region.
High-intensity shock waves have been used for years to pulverise urinary stones so that they are small enough to be passed out naturally.
Only about 18 per cent of this energy is needed to treat erectile dysfunction (ED), said senior consultant urologist Peter Lim, who runs a practice at Gleneagles Hospital.
The energy is used to cause very minor injuries to genital tissues to stimulate growth factors - proteins that prompt more blood vessels to grow.
At least three companies have machines to deliver low-intensity shock waves for ED, but the one by Israeli company Medispec is the most well-studied, according to doctors.
The Health Sciences Authority classified it as a low- to medium-risk device in class B in November last year, said Mr Steven Ho, general manager of its distributor, Radiance Medical Systems. Devices such as hypodermic needles and suction equipment also belong to this class.
The machine was brought into Prof Lim's clinic in March this year so he could demonstrate the therapy on three patients during a training course at the annual scientific meeting of the Singapore Urological Association.
Since then, Prof Lim has carried out the shock wave treatment on 85 men, while two other urologists have used the same machine on four men.
Two private clinics have purchased the machine. Dr Simon Chong, a consultant urologist at Pacific Healthcare Specialist Centre at Paragon, has lined up two patients for the therapy, while Sincere Medical Specialist Center for Men is awaiting its arrival.
Doctors at Khoo Teck Puat Hospital (KTPH) and National University Hospital agreed that the concept is sound and plan to conduct their own studies.
Dr Colin Teo, head of KTPH's urology department and president of the Society for Men's Health, said new ED guidelines to be launched next month recommended that its use be limited to specialists only.
Effectiveness of therapy
During sexual arousal, the brain tells nerves in the pelvis to release neurotransmitters that stimulates blood vessels in the penis to dilate and the smooth muscles to relax. All this increases blood flow into the organ for it to become rigid.
Erectile dysfunction (ED) can be caused by poor blood supply to the genitals, nerve damage, hormonal imbalance and even psychological barriers.
Shock wave therapy is suitable for men with vascular diseases, such as atherosclerosis (hardening of the arteries) which, in turn, result in poor blood supply to their genitals.
Seven in 10 of all ED patients would be suitable for this treatment, said senior consultant urologist Peter Lim, who runs a practice at Gleneagles Hospital.
Dr Joe Lee, a consultant at National University Hospital's department of urology, said many studies have linked ED to cardiovascular disease, since both share similar risk factors of diabetes, high cholesterol, smoking, obesity, hypertension and ageing.
"When blood vessels in the body become blocked or narrowed, those in the penis are not spared either," said Dr Lee.
Mr Tan (in the main story) suffers from diabetes, ischaemic heart disease and hypogonadism, also known as testosterone deficiency syndrome, so it is perhaps not surprising that he also suffers from ED.
Doctors said the shock wave therapy can be offered to men with varying degrees of ED at different treatment points, helping to plug existing gaps in treatment.
Dr Ho Siew Hong, medical director of the Sincere Medical Specialist Center for Men, thinks it may help the 20 per cent of men who have such severe dysfunction that they do not respond to phosphodiesterase-5 (PDE-5) inhibitors at all.
Of the rest who do respond, half may find the drugs ineffective after five to 10 years, when any underlying vascular disease worsens, he said.
In addition, those who are taking nitrate-containing drugs for heart conditions cannot take PDE-5 inhibitors because the drugs, when taken in combination, can lower blood pressure to a potentially life-threatening level.
And up to 10 per cent of those who repeatedly use injections may develop scar tissue and a condition called Peyronie's disease, where the erect penis deviates to one side.
For all these men, shock wave therapy, which has no reported side effects so far, may be useful.
Promising results are being reported. One year of study of 20 men with mild to moderate ED showed that six months after treatment, 10 still had spontaneous erections and did not require PDE-5 inhibitors.
In another study, 29 men who had severe ED, multiple cardiovascular risk factors and who responded poorly to oral drugs were tracked after receiving shock wave therapy. Two months later, 21 men could have sex with PDE-5 inhibitors while eight did not require any medication.
Prof Lim has analysed the results of 52 men who had the therapy here and said 43 were able to have sex successfully on their own afterwards.
The treatment also appeared to reduce the condition's severity in 35 men, as they responded to oral drugs, when they previously did not.
None reported pain or side effects during or after treatment.
With shock wave therapy being relatively more expensive, doctors are unsure it will be as well received as the oral drugs.
At Prof Lim's clinic, one cycle of shock wave treatment - comprising six sessions over three weeks - costs about $3,600.
However, a box of four PDE-5 inhibitors - each taken prior to sex - costs between $80 and $100; while a month's supply of daily, low-dose PDE-5 inhibitors costs $200, said Dr Simon Chong, a consultant urologist at Pacific Healthcare Specialist Centre at Paragon. Each penile injection costs between $30 and $80, he added.
But doctors are hopeful that the latest treatment option will take off.
Said Dr Chong: "If you think about it, you cannot put a price on the ability to have an erection whenever you want to, without having to take a pill or have an injection."