Police have only just revealed the discovery last month of fake "Captagon" pills and say they do not know if they were destined for the Middle East.
Meanwhile, Italy's financial police revealed this week they had uncovered 37.5m pills of another drug, tramadol, also used by Islamist fighters.
The cargo was heading for Libya.
Tramadol is a synthetic opioid-like drug used as a painkiller.
Italian police said the consignment had come from India and would have been used for two purposes: to help finance Islamist terrorism and for use by jihadist fighters as a stimulant and to heighten resistance to physical stress.
Italian police said the drugs could have raised €75m (£63m; $81m) on the open marke
Abuse of tramadol was described by a report this year as "rampant" in the ranks of Boko Haram fighters in Nigeria and to have played a role in "enabling atrocities on both sides of the conflict".
Fake Captagon and the Syrian conflict
The main ingredient of counterfeit Captagon tablets is usually amphetamine but illegal drug-makers often add caffeine or other substances.
It is widely seen as fuelling the conflict in Syria, as the drug has generated millions of dollars in revenue and has been used by combatants. The drug previously sold as Captagon was a psychostimulant called fenethylline, banned since the 1980s, related to amphetamine.
Taking the drug lowers inhibitions to violence and prolongs a fighter's ability to remain alert for combat.
The counterfeit Captagon pills were found in a shed in Brunssum in the southern Dutch province of Limburg, along with a tablet-making machine and other drugs.
Police said the tablets had been analysed by the Netherlands Forensic Institute and found to contain two ingredients, amphetamine and caffeine. The pills were of a type used as a stimulant in the Middle East but it was not known if that was their destination.
A man who lived in the neighbouring house was arrested a few days after the discovery last month and a woman was detained on Monday. Two other suspects are being sought.
In March, Greek police arrested four suspects in a fake Captagon-making ring and confiscated 650,000 tablets.