John Martin, 48, denies murdering Russian pianist Natalia Strelchenko, 38, at their Manchester home on their second wedding anniversary last August.
Manchester Crown Court heard he took the diazepam believing it to be his prescribed anti-depressant medication.
He said "everything is very blurry".
It is alleged the double bass player strangled and beat Ms Strelchenko, who was also known by the surname Strelle, in a loss of temper at their home in Newton Heath on 30 August.
Giving evidence, Mr Martin, also known as Jon Skogsbakken, said he had been suffering with depression and that he had unwittingly taken unmarked diazepam tablets for around six weeks.
He said he "guessed" they had been prescribed to his wife.
Mr Martin told the jury he had suffered with depression since 2005 and his GP would prescribe him Escitalopram.
He told the court that taking the diazepam was "the biggest mistake of my life".
Mr Martin said: "I would never have taken the two together, absolutely not", if he was aware the tablets could have a "paradoxical and aggressive response" and when taken with alcohol it could increase the effects.
The court heard following an argument with Ms Strelchenko, he drank four cans of cider at their home before going to the working men's club where he had another pint of cider and then returned home.
He said: "I just sat down on the floor and poured myself a glass of wine. I started to drink it. Natalia was angry because I was drinking wine and she tried to take it away from me.
"Everything is very blurry for me. I can't recall anything after that point.
"I still love her very much. I would never wish to do such terrible things to her. I recall virtually nothing."
Mr Denney QC said: "Do you accept that on evidence you killed Natalia?"
Mr Martin replied: "Well according to what the witnesses described...I perfectly understand that I must be the man behind it."
Mr Martin, who had worked for the computer firm IBM in his home country of Norway, met Ms Strelchenko in 2007.
She had performed piano recitals at New York's Carnegie Hall and London's Wigmore Hall.
The trial continues.

It follows an incident which saw a S92 spin more than 180 degrees on landing.
One-off inspections of the tail rotor on all S92 helicopters around the world took place last week, following the incident on the West Franklin platform in December.
Operators are now being told to increase inspections on the aircraft's tail rotor.
No-one was injured in the incident, but there was significant damage to the deck of the platform.
As a result of the initial safety checks, 11 tail rotor bearings were returned to manufacturer Sikorsky for further tests.
The Federal Aviation Administration has now said further inspections using specialist cameras must be carried out every 10 hours of flying time.
North Sea operators said they were fully complying with the latest directive.

Hong Kong's Hang Seng index closed up 0.2% at 21,914.40, while the Shanghai Composite ended down 0.4% at 3,539.18.
The index was 9.4% higher for the year, making it one of the best performing markets globally.
Investors were also holding off ahead of manufacturing data due out on Friday.
The data should shed light on the extent of the slowdown in the world's second largest economy.
Economists expect the manufacturing surveys to show factory activity shrinking again in December.
Several markets closed early on New Year's Eve such as Hong Kong and Australia, while Japan and South Korea were closed for public holidays.
In Sydney, the benchmark S&P/ASX 200 index closed down 0.5% to 5,295.9, but lost more than 2% for the year.
That marked the index's first annual fall since 2011 as slumping iron ore and metal prices hit blue chip mining stocks, while new rules hurt the banking sector.
Sliding oil prices hovering around 11-year lows capped gains in energy-related stocks in Australia and across the region.
"The correlation between moves in the oil price and equity markets are becoming increasingly concerning and do not bode well for today's Asia session," said market strategist Angus Nicholson at trading firm IG in a note.
Crude oil prices held losses after falling 3% overnight.
A musician accused of murdering his wife has told a court "I perfectly understand that I must be behind it" but recalls "virtually nothing" after taking diazepam and alcohol.
[NEXT_CONCEPT]
Additional checks are being carried on a North Sea helicopter fleet in the wake of safety fears.
[NEXT_CONCEPT]
Chinese stocks were mostly lower on the last day of 2015 in thin trading with some of Asia's biggest markets closed ahead of the new year.